Temperature Dependence of Si-Based Thin-Film Solar Cells Fabricated on Amorphous to Microcrystalline Silicon Transition PhaseKobsak SRIPRAPHA Ihsanul Afdi YUNAZ Seung Yeop MYONG Akira YAMADA and Makoto KONAGAI Department of Physical Electronics, Tokyo Institute of Technology, 2-12-1-S9-9, O-okayama, Meguro-ku, Tokyo 152-8552, Japan Quantum Nanoelectronics Research Center, Tokyo Institute of Technology, 2-12-1-S9-9, O-okayama, Meguro-ku, Tokyo 152-8552, Japan (Received June 5, 2007; accepted August 20, 2007; published online November 6, 2007) The temperature dependence of silicon (Si)-based thin-film single-junction solar cells, whose intrinsic absorbers were fabricated on the transition phase between hydrogenated amorphous silicon (a-Si:H) to hydrogenated microcrystalline silicon (mc-Si:H), was investigated. By varying the hydrogen dilution ratio, wide-band-gap protocrystalline silicon (pc-Si:H) and mc-Si:H absorber layers were obtained. Photo-current density –voltage (Photo-J–V) characteristics were measured under AM1.5 illumination at ambient temperatures in the range of 25 –75 C. We found that the solar cells with pc-Si:H, which exists just below the a-Si:H to mc-Si:H transition boundary, showed the lowest temperature coefficient (TC) for conversion efficiency and open-circuit voltage (Voc), while the solar cells fabricated at the onset of the a-Si:H to mc-Si:H phase transition exhibited a relatively high TC for and Voc. Experimental results indicated that pc-Si:H is a promising material for the absorber layer of the single junction or the top cell of tandem solar cells that operate in high temperature regions. KEYWORDS: temperature dependence, amorphous silicon, protocrystalline silicon, Si thin-film solar cell, solar cells1.IntroductionIn general, the solar cell performance is measured under the standard test conditions (STC) of a cell temperature of 25 摄氏度and an irradiance of 100mWcm 2 with AM 1.5 spectral distributions. However, in an outdoor installation, the operating temperature of solar cells considerably changes depending on the environment, i.e., the climate in the installed area.In a tropical region, the operating temperature often reaches more than 70 摄氏度. The increase in the operating temperature leads to a decline in conversion efficiency mainly due to the drop in open-circuit voltage(Voc) Among Si-based solar cells, bulk crystalline Sisolar cells which include single-crystalline-Si (c-Si) and polycrystalline-Si (poly-Si) solar cells show higher than thin-film solar cells at room temperature. However, ηof c-Siand poly-Si solar cells seriously decreases with an increase in the operating temperature, while hydrogenated amorphous Si (a-Si:H)-based thin-film solar cells exhibit relatively small variation in.The main reason for the lowertemperature coefficient (TC) of a-Si:H-based solar cells is their wide-band-gap intrinsic absorber or high Voc compared with those of bulk crystalline Si-solar cells. Taking the real output power affected by the operating temperature and production cost into account, a-Si:H-based thin-film solar cells have advantages over bulkcrystalline-Si solar cells for use in high temperature areas such as a tropical region. However, it is well known that a-Si:H-based thin-film solar cells exhibit light-induced degradation after light exposure, the so-called Staebler–Wronski effect (SWE).The SWE in a-Si-based thin-film solar cells is also a veryim portant factor that must be considered for outdoor installation. During the past 30 years, extensive research has been conducted to suppress the SWE. As a result, two kinds of edge materials near the phase boundary have been developed as stable intrinsic absorbers: one is the wideband- gap protocrystalline silicon (pc-Si:H) existing just below the a-Si:H-to-microcrystalline silicon (mc-Si:H) transition transition and the other is the narrow-band-gap mc-Si:H with crystalline silicon volume fraction (Xc) of 30 –50% obtained near the onset of the phase transition. The pc-Si:H material nucleate from the deposition of the a-Si:H at just before the transition boundary of a-Si:H to a-Si:H t mc-Si:H mixedphase. Once, the a-Si:H t mc-Si:H transition is detected,which can be observed by a real time spectroscopic ellipsometry (RTSE), the growing material is no longer considered pc-Si:H.10) The unique properties of pc-Si:H are the optical band gaps (Eopt) and the Urbach tail. The Eopt of pc-Si:H is larger than conventional material and increases with increasing H2 dilution ratio. Besides, the narrower Urbach tail in pc-Si:H causes the higher hole drift mobility than conventional materials. The key feature of the pc-Si:Hmaterial is its relative stability to light induced degradation as observed in the electron-mobility lifetime product and similarly in the solar cell fill factor. These two kinds of materials are attractive for application to Si-based thin-filmsolar cells because of their low SWE.Although the pc-Si:H solar cell has shown a good temperature dependence among Si-based thin-film solar cells,the behavior of TC for pc-Si:H solar cells has not yet been clarified. In this work, we investigated the temperature dependence of a-Si:H-based solar cells fabricated in the pc-Si:H to mc-Si:H transition regime. The TC values after lightinduced degradation were also investigated in order to findthe optimal absorber layer for the use at high operating temperatures.2.Experimental ProcedureThe p–i–n single-junction solar cells were fabricated on Asahi U-type glass substrates in a multi chamber system with the structure of glass/SnO2:F/hydrogenated p-type amorphous silicon carbide (p-a-SiC:H)/buffer/intrinsic (i-)absorber/n-type amorphous silicon (n-a-Si:H)/boron-doped zinc oxide (ZnO:B)/Ag/Al with the cell area of 0.086 cm2.The thicknesses of p, buffer, i-, and n-layers were kept constant at around 12, 4, 320 – 340, and 2 nm, respectively.All solar cells were fabricated at the substrate temperature of around 200 C with deposition pressures of 50 – 70 Pa. Thevery high frequency (60 MHz) plasma-enhanced chemical vapor deposition (VHF-PECVD) was used to deposit the i-layer. The i-layers were deposited at different silane concentrations, SC ? SiH4=e SiH4 t H2T, by varying SC from 6.0 to 2.4% in order to obtain material with the phase transition from amorphous to microcrystalline silicon.With a decrease in SC, the deposition rate of the i-layerdeclined from 1.6 to 0.9. The doped (p- and n-layers) and buffer layers were deposited by a radio-frequency(13.56MHz) PECVD technique. ZnO was deposited by metal organicchemical vapor deposition (MOCVD) as a back reflector, while Ag and Al were evaporated as back electrodes for all samples.The Raman spectroscopy was performed using a JASCONRS-1000 system with a semiconductor laser at a wavelength of 532 nm. Ex-situ spectroscopic ellipsometry (SE)measurements (J. A. Wollam) were used with a variableangle spectroscopic ellipsometer. The temperature dependence of the solar cell parameters were measured using a solar simulator in a chamber at ambient temperatures (T) in the range of 25 –75 C with a step increment of 10 C under 1-sun (AM1.5, 100mW cm2) irradiation. The temperature of the sample was regulated by a temperature-controlled airflow. The temperature dependence of solar cells was obtained from photo-current density–voltage (photo-J–V) measurements. The value of TC can be expressed aswhere Z denotes the solar cell parameters, i.e.,η, Voc, shortcircuit current density (Jsc), and fill factor (FF). The normalized temperature Tn is chosen to be 25 C because it corresponds to the standard reference condition for solar cell measurement. The 1-sun standard light-soaking test was performed in a climate chamber at the temperature of 50 C for 100 h.3.Results and Discussion3.1 Characterization of intrinsic absorbersIn the first series of experiments, we inspected the Raman spectra for the solar cells fabricated in the phase transition regime. Figure 1 shows the Raman spectra for solar cells prepared with different SCs which are measured from the nside(rear-side of the solar cells). The Raman spectra were deconvoluted to four Gaussian peaks centered at the Raman shift areas at around 430, 480, 510, and 520 cm1, which correspond to the longtitude optical (LO) mode of a-Si:H, the transverse optical (TO) mode of a-Si:H, the defective crystalline phase and the TO mode of c-Si, respectively. The defective part of the crystalline phase is included in thecrystalline fraction.15) The Xc calculated from the Raman spectrum is expressed aswhere Ii is the area under the Gaussian peak centered at the Raman shift of i cm 1 and I480 t I510 t I520 is the total integrated area. By decreasing SC from 6.0 to 4.0%, the peak position of Raman spectra of a-Si:H in the TO mode increased from the Raman shift of 475 to 480 cm1, as shown by the solid line in the figure, which means that the a-Si:Hmicrostructure improved when SC decreased, leading to further improvement of stability against illumination.16) For SC ?3:2%, the Raman spectra exhibited two peaks at 480 and 517 cm1, which correspond to the onset of mc-Si:H growth. With further decrease of SC, SC < 3:2%, the peak position of Raman spectra became that of c-Si inthe TO mode at 520 cm 1. The Xc of the films deposited with SC at4.0, 3.2, 2.8, and 2.4% were 0, 18, 43 and 52%, respectively.In order to measure Eopt, i-layers were deposited on p-a-SiC:H films. The p-a-SiC:H and i-layers were deposited under the same conditions as those for solar cells on Corning glass 7059. Unfortunately, we could not measure Eopt of the films deposited at SC 3:2%, because they had a tendency to peel off, despite the underlying p-a-SiC:H. An ellipsometric spectrum e ;T was measured by SE over the range of 1.2 to 4.1 eV. The three-layer model of p-a-SiC:H,i-absorber and surface roughness layers was used to fit the experiment data. The Tauc–Lorentz empirical model wasemployed to derive optical constants for p-a-SiC:H and i-absorber.17) The absorption coefficient () data obtained by fitting the i-layers was used to calculate Eopt by means of the indirect-band-gap Tauc model.18) Figure 2 shows Eopt of i-layers deposited with different SCs. When SC was decreased from 6.0 to 4.0%, Eopt slightly increased from 1.87 to 1.92 eV (as indicated by the solid line). We conclude that, from their large values of Eopt, the films deposited with SC beyond 4.0% are pc-Si:H materials. Eopt of mc-Si:H is normally lower than that of typical a-Si:H (1:7 eV), and itcan be assumed that with decreasing SC, Eopt of mc-Si:Happroaches that of c-Si.3.2 Temperature dependence of solar cellsThe initial performances at the temperatures of 25 and 75 C for the solar cells fabricated at the phase transition from a-Si:H to mc-Si:H are summarized in Table I. It was found that, with increasing T, Jsc progressively increased. There are many reasons to explain the increase in Jsc, for example, the reduction of Eopt of the semiconductor, the increase in the diffusion length of the minority currentcarriers,20,21) and the reduction of the recombination velocity or unfilled localized states.1,22) In contrast, Voc decreased at higher temperature. This behavior could be attributed to the increase in the reverse saturation current and the decrease in Eopt.2,19–22) FF also decreased with increasing T, except in the case of SC ?2:4%. This condition is the only one that showed a positive value for FF. In general, the decline of at high temperature mainly originates from the drop of Voc and FF.Jsc showed a slight decrease with the decrease of SC from 6.0 to 4.0%. This is probably due to the wide Eopt of pc-Si:H. With a further decrease in SC, Jsc increased because of the reduction in Eopt. It should be noted that the lowest Jsc was obtained at the onset of mc-Si:H. With decreasing SC from pc-Si:H (SC > 3:2%) to the onset of mc-Si:H solar cells (SC ?3:2%), Voc and FF both decreased and drastically fellwhen the films approached the mc-Si:H region (SC < 3:2%),resulting in the consecutive decline in the . The highest initial of 9.44% was obtained from a solar cell fabricated at SC of 6.0%. It was also found that Jsc and FF were less sensitive to T than Voc. The temperature dependence of solar cell parameters is shown in Table I. Figure 3 shows the normalized solar cell parameters in the initial state. In general, FF of a Si-based solar cell decreases when T becomes high, mainly because of the reduction of Voc. However, it has been reported that for a-Si:H-based thin-film solar cells, FF sometimes increases with increasing T.This is probably due to the decrease of the contact resistance or the increase of the mobility-carrier lifetime product within the collection regionin a-Si:H solar cells. It was concluded that the increase incollectionlength with T was large in a-Si:H solar cells with poor transport properties.As shown in Fig. 3, normalized Voc for the pc-Si:H solar cell (SC between 6.0 and 4.0%) exhibited less degradation with the increase of temperature. Voc also drastically fell when SC reached 2.4%, because of the strong influence of the increase in Xc. FF and also showed similar behavior to Voc. It should be noted that although the solar cell fabricated at SC of 2.4% had the largest drop in normalized Voc, because of the positive change in FF, normalized was not the lowest. The increase of Jsc for solar cells with SC < 4:0% (above the onset of mc-Si:H) was larger than that of pc-Si:H solar cells. This behavior can be ascribed to the reduction of Eopt with increasing SC. According to our experimental results, the lowest TC for around 0:242%/ C was achieved in the pc-Si:H solar cell fabricated at SC of 5.0%.The dark-J–V characteristics for the fabricated solar cellslisted in Table I were investigated. We found that, the dark-current density before and after the measurement of the temperature dependence remained the same. The reverse saturation current-density (J0) and diode quality factor (n) of pc-Si:H solar cells showed low values compared with solar cells fabricated under the onset and mc-Si:H conditions. The lowest J0 and n were obtained at SC of 5.0%, and their values were 6:7 1013 A cm 2 and 1.6, respectively. It should be noted that the solar cells fabricated at the onset of mc-Si:H exhibited drastic increases in J0 and n. The values of n of the solar cells fabricated with SC 3:2% were larger than 2.0.In order to investigate the effect of light soaking on the temperature dependence behavior, all solar cells were illuminated under 1-sun for 100 h and measured again. Figure 4 displays the normalized solar cell parameters after light soaking. The normalized TC for Voc and Jsc showed similar temperature-dependent trends as those in the initial state. The normalized FF for solar cells with pc-Si:H and onset of mc-Si:H (SC 3:2%) showed lower TC than that before the degradation state. It should be noted that the solar cells fabricated at SC of 2.8% showed positive TC for FFafter light soaking. At present, the reverse behavior of FF after light soaking of this solar cell cannot be clearly explained. The normalized of all cells decreased with rising T, except in the case of solar cells fabricated at SC of 2.8%, which showed a positive TC value because of the influence of FF.TCs of solar cell parameters determined before and after light soaking are summarized in Table II. It should be noted that TC for Voc was inversely proportional to initial Voc in Table I. After light soaking, TC for Jsc became higher, except in the case of solar cells fabricated near the onset of mc-Si:H (SC ?4:0%) where a small decrease in TC for Jsc was observed. TC for Voc of the solar cells fabricated in thepc-Si:H regime also displayed a small increase after light soaking, while that of mc-Si:H solar cells decreased. TC for of all solar cells after light soaking exhibited a small decrease, except in the case of the solar cell fabricated at SCof 2.8%. TC for FF also showed a similar tendency as that for . It should be noted that, although the solar cell that was fabricated at SC of 2.8% showed positive TC for mainly because of positive TC for FF, the initial and FF of this cell were low. Consequently, we did not take this result into consideration.From the results, it can be concluded that the pc-Si:H solar cells presented a goodtemperature dependence as well as a reasonable conversion efficiency and low light-induced degradation compared with solar cells fabricated under the onset and mc-Si:H conditions. As shown in Fig. 5, the lowest TC for before and after light soaking was obtained at SC of 5.0%, and their values were 0:242 and 0:214%/C, respectively. Therefore, pc-Si:H fabricated under this condition is promising for use in a single cell or as the top cell in a tandem cell operating at high temperatures. In order to apply this film to tandem cells, the temperature dependence of solar cells fabricated at SC of 5.0% with different thicknesses was also determined. As shown in Fig. 6, in the initial state, TC for decreased with the increase of i-layer thickness because of the increase of the initial efficiency. On the other hand, after light soaking, solar cells with thinner absorber showed a low TC for because of the lower total defect density.In addition, pc-Si:H/mc-Si:H double-junction silicon solar cells were fabricated using ZnO:B with the thickness of around 50 nm as intermediate layer between the pc-Si:H top cell and mc-Si:H bottom cell. Both absorber layers were deposited by VHF-PECVD. The thicknesses of the pc-Si:H top cell and mc-Si:H were 170 and 2000 nm, respectively. The typical initial conversion efficiency of this double-junctionsilicon solar cell was 11.76% with Jsc of 11.5 mA cm2, Voc of 1.38V and high FF of about 0.75. The TC and lightinduced degradation behavior of pc-Si:H/mc-Si:H doublejunction silicon solar cells are now being investigated.4.ConclusionsThe temperature dependence of pc-Si:H and mc-Si:Hsingle-junction silicon solar cells fabricated near the phasetransition regime by VHF-PECVD was investigated. By decreasing SC from 6.0 to 2.4%, the absorber layerproperties were changed from those of pc-Si:H to the mc-Si:H phase. The wide-band-gap pc-Si:H (Eopt1:9 eV) was obtained at SC > 3:2%, while the onset of mc-Si:H and the narrow-band-gap mc-Si:H were deposited at SC ?3:2% and SC < 3:2%, respectively. It was found that the values of TC for were inversely proportional to the initial Voc. Our experimental results indicated that the pc-Si:H solar cells had TC for around 1.5 – 2.0 times lower than that of solar cells fabricated with the phase of the onset and mc-Si:H. Thesolar cells fabricated with the onset of the mc-Si:H and mc- Si:H phase (SC 3:2%) showed fluctuations of TC for FF and after light soaking, while pc-Si:H solar cells exhibited similar TC trends for all photovoltaic parameters. It was found that the lowest TC for was obtained from the pc- Si:H solar cell fabricated at SC of 5.0%. Since the pc-Si:H solar cell exhibits a good light-induced stability and low temperature dependence, it is advantageous for use in a tropical region.。