Light-Soaking Effects on the Open-Circuit Voltage of a-si:h Solar Cells

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1 Syracuse University SURFACE Physics College of Arts an Sciences 2005 Light-Soaking Effects on the Open-Circuit Voltage of a-si:h Solar Cells Jianjun Liang Syracuse University Eric A. Schiff Syracuse University S. Guha Unite Solar Ovonic Corporation Baojie Yan Unite Solar Ovonic Corporation J. Yang Unite Solar Ovonic Corporation Follow this an aitional works at: Part of the Physics Commons Repository Citation "Light-soaking Effects on the Open-circuit Voltage of Amorphous Silicon Solar Cells," J. Liang, E. A. Schiff, S. Guha, B. Yan, an J. Yang, in Amorphous an Nanocrystalline Silicon Science an Technology , eite by R. Collins, P.C. Taylor, M. Kono, R. Carius, R. Biswas (Materials Research Society Symposium Proceeings Vol. 862, Pittsburgh, 2005), A13.6 This Conference Document is brought to you for free an open access by the College of Arts an Sciences at SURFACE. It has been accepte for inclusion in Physics by an authorize aministrator of SURFACE. For more information, please contact surface@syr.eu.

2 Mater. Res. Soc. Symp. Proc. Vol Materials Research Society A Light-Soaking Effects on the Open-Circuit Voltage of a-si:h Solar Cells Jianjun Liang, 1 E. A. Schiff, 1 S. Guha, 2 B. Yan, 2 an J. Yang 2 1 Department of Physics, Syracuse University, Syracuse, NY USA 2 Unite Solar Ovonic Corp., Troy, MI USA ABSTRACT We present measurements on the ecline of the open-circuit voltage V OC in a-si:h solar cells uring extene illumination (light-soaking) at 295 K. We use a near-infrare laser that was nearly uniformly absorbe in the intrinsic layer of the cell. At the highest photogeneration rate (about 2x10 21 cm -3 ), a noticeable ecline (0.01 V) occurre within about 10 minutes; V OC stabilize at 0.04 V below its initial value after about 200 hours. We foun that both the kinetics an the magnitues of V OC are reasonably consistent with the preictions of a calculation combining a bantail+efect picture for recombination an a hyrogen-collision moel for efect generation. The version of the hyrogen-collision moel that we use assumes that only bantail recombination rives the hyrogen collision processes. Within this picture, the crossover between bantail an efect recombination occurs on the same timescale as the light-inuce annealing process that accounts for stabilization of the optoelectronic properties for long lightsoaking times. INTRODUCTION The metastable egraation of the optoelectronic properties of a-si:h has been actively stuie for more than 25 years. Important progress has been mae by many researchers, but a fully satisfactory picture for the effect has never emerge. The ifficulty of the metastability problem may, at least in part, be ascribe to the fact that there are probably two ifficult, linke problems that must be unerstoo simultaneously. First, it is likely that photocarrier recombination rives metastability, but - even for a single light-soaking state of an a-si:h sample - no consensus viewpoint on these recombination processes has emerge. Secon, even if one accepts the role of recombination in meiating metastability, there is no consensus viewpoint about how recombination leas to egraation by efect generation or other metastabilities. The present paper exploits the close connection between the open-circuit voltage V OC measure in an nip solar cell an photocarrier recombination in the bulk, intrinsic-layer material. In particular we report a series of measurements of the time-epenent egraation of V OC in Unite Solar cells uner near-solar illumination. In recent work [1] we reporte the temperatureepenence of V OC at solar illumination intensities. We conclue from these stuies that, for their as-eposite state, the recombination traffic in our samples flowe preominantly through the valence bantail states. For the stabilize, light-soake state recombination traffic appears to be split fairly evenly between the valence bantail an efects. We emphasize that these conclusions apply to our materials, uner near-solar illumination, an near room-temperature; these are, of course, the conitions of greatest practical significance for solar cells. Using this recombination moel as the starting point, we have been able to account for the time-epenent ecay of V OC using a straightforwar extension of the hyrogen-collision moel [2] for efect generation; the extension assumes that mobile hyrogen is generate by bantail recombination. For our materials, we can also exclue the best known alternative moel [3]. There is at least one implication of this perspective on metastability that eserves further investigation. It appears that there is a coincience of two time-scales: (i) the time at which

3 A crossover between bantail an efect recombination occurs, an (ii) the time at which a stabilize state of the sample is achieve. The coincience suggests that efect recombination may meiate light-inuce annealing. MEASUREMENTS For these experiments, six epositions of nip solar cells on stainless steel substrates were one at Unite Solar Ovonic Corp.. The n an p layers were the same in all epositions; the eposition time for the intrinsic layer was chosen to give intrinsic layer thicknesses from 185 nm to 893 nm. The cells were not optimize for solar conversion efficiency, but the iniviual layers are comparable to those use in highefficiency cells. Details of the eposition proceures have been given elsewhere [4]. As-eposite properties of the cells were measure uner a solar simulator. This paper emphasizes results for the thickest cells, for which the white-light V OC average 0.98 V an the white-light fill factors average Further stuies were one using a 30 mw, 685 nm wavelength near-infrare laser. We chose to use this laser because its wavelength is absorbe fairly uniformly throughout the intrinsic layer of the cells, which substantially simplifies moeling of the measurements. We were able to achieve photocurrent ensities in the cells that were comparable to solar illumination. Measurements of the ecline of the open-circuit voltage V OC are presente in Fig. 1. The ifferent symbols inicate measurements for four ifferent laser intensities. We use four ifferent cells on the same substrate for these measurements; the cells ha very similar initial properties uner white-light. The average photogeneration rates G for each experiment are inicate in the figure; these were calculate from the measure photocurrent ensity J p at ( ) e V = -2 V using the expression G = J V, where e is the electronic charge an is the p Open-circuit voltage (V) x10 21 cm -3 s x x x Light-soaking time (h) Figure 1. Decay of the open-circuit voltage with time at four ifferent intensities; the average photogeneration rate in the intrinsic layer is inicate. The symbols enote the experimental measurements in four cells (893 nm i-layer thickness, laser wavelength 685 nm) on the same substrate. The lines represent calculations for the extene hyrogen-collision moel escribe in the text. thickness of the intrinsic layer. The measurements for the highest intensity are most comparable to solar illumination; the photocurrent ensity was 18 ma/cm 2 with the laser (an -2 V), an was about 15 ma/cm 2 with the solar simulator (short-circuit). We maintaine the sample holer temperature at K to within better than 0.1 K. An interesting feature of the measurements is that the onset of egraation occurs for later times at lower photogeneration rates. If we efine an onset time when V OC has ecline by 0.01 V, it is evient in Fig. 1 that this onset time increases by more than 1000 over the range of

4 A photogeneration rates in the figure. For the largest photogeneration rate it appears that V OC is approaching a saturation value within this time winow; saturation at longer times is fairly wellestablishe in earlier work on solar cell parameters, efect ensity, an other optoelectronic properties of a-si:h [5,6]. A MODIFIED H-COLLISION MODEL In another paper [1] we presente temperature-epenent measurements on the open-circuit voltage for a-si:h cells from the same substrate use for Fig. 1. These measurements for the aseposite an the light-soake states are consistent with the following moel. For the aseposite states of a-si:h, the photocarrier recombination traffic is preominantly through the valence bantail (through the process of electron capture by a bantail-trappe hole). Lightsoaking then has a surprisingly moest effect: the efect-ensity rises until recombination traffic is fairly evenly split between the bantails an the efects. One important fact that has been establishe in several light-soaking experiments [3,5,6,7] is 3 that the efect ensity increases as about the 1/3 power of the light-soaking time: N t 1 from shortly after the onset of illumination until a saturate state is achieve. The original SJT explanation [3] for the t 1/3 form mae two assumptions. First, recombination traffic through the bantails R t generate efects: N R. (1) t t Secon, for times at which the t 1/3 form applie, most recombination traffic went through efects R G. The particular moel that was propose ha the property that the recombination traffic R t through the tails obeye: R G, (SJT recombination) (2) ( ) 2 t N where G is the carrier photogeneration rate an N is the ensity of efects. With this assumption, one obtains the form usually foun by experiment: ( t, G) 3C G 2 t N = SW. (3) At least for the samples we have stuie, SJT-recombination oes not apply over a significant time-regime. Recombination traffic mostly passes through the bantail states ( R t G ) in the as-eposite state, an falls only moestly even for the saturate, light soake state. Although we have not stuie the growth of the efect ensity uring light-soaking with these particular samples, the near-universality of the observation of the t 1/3 form for N (t) suggests that we seek another explanation than SJT-recombination for the kinetics of light-soaking. A similar situation also applies to light-soaking with intense pulses, or at very low sample temperatures. Bantail recombination is completely ominant, but efect growth nonetheless shows a sublinear epenence on illumination time [8,9]. The principal alternative to the SJT approach is the hyrogen-collision moel [2], which assumes the existence of an illumination-inuce mobile hyrogen ensity H m obeying: H G N. (4) m H G H is the rate at which light generates mobile hyrogen, an it has been assume that the

5 A recombination traffic of hyrogen mostly flows through angling bons (ensity N ). Rarely, mobile hyrogens collie to form a metastable, paire hyrogen complex. Such a collision leaves 2 metastable angling bons that are not present in the initial state; the efect generation rate obeys the hyrogen-collision proportionality: N t ( G N ) 2 Hm 2 H. (hyrogen-collision) (5) Note that the empirical G 2/3 t 1/3 form for N (t) (eq. 3) obtains if we assume that The assumption G H G nees to be consistent with several experiments [10] 0.98 Initial that suggest that efect generation is meiate by photocarrier recombination, an most probably by the tail state 0.96 recombination GH Rt. This ientity is broaly consistent with the conclusion for our samples that R t G is of orer unity through most of the light-soaking process in our samples, an we therefore use the more irect form for the hyrogen-collision moel: N 2 GH Rt H m t N N or, collecting several proportionality constants together as C SW : 2 ( N ) 2 N Rt = C SW (6) t N R t epens upon the efect ensity. The equation R t = G applies as long as the efect ensity is sufficiently small that it oesn t significantly that its effect on recombination remains small. For this conition, the G 2/3 t 1/3 form obtains. BANDTAIL+DEFECT CALCULATIONS In another paper in these proceeings [1] we present the parameters an proceures of a bantail+efect calculation of the open-circuit voltage; the calculation use the AMPS-1D computer program ( Pennsylvania State University). In Fig. 1 we have presente (as the four 2, Open-circuit VoltageV oc (V) Rate (10 21 cm -3 s -1 ) Measure Saturation Generation G H G. Bantail Recombination Defect Recombination Defect Density N (10 15 cm -3 ) Figure 2. Bantail recombination rate R t an open-circuit voltage V OC calculate as a function of efect ensity N using the bantail+efect coe for uniform photogeneration G = cm - 3 s -1. The photogeneration rate an efect recombination rate are also shown. The initial experimental value of V OC correspons to N = 1x10 15 cm -3, an we have suggeste a saturation value. Note that bantail recombination is comparable to G throughout this range of efect ensities.

6 A lines) the corresponing calculations for V OC (t) base on eq. (6). We use the following proceure for these calculations. Assuming a spatially uniform efect ensity N, we use the bantail+efect calculation to calculate the open-circuit voltage V OC as a function of N for each of the four generation rates; the results are illustrate in Fig. 2 for G = cm -3 s -1, along with the associate rates of bantail recombination R t an efect recombination R evaluate at the mile of the cell. We use the function R t (N ) to numerically integrate eq. (6) to obtain N (t); we use the value C SW = 10 cm -3 s that is typical of irect stuies of the efect generation uring light-soaking [5,6]. We arbitrarily chose N (0) = cm -3. Once N (t) has been compute, V OC (t) is obtaine from Fig. 2, an plotte in Fig. 1 for the four ifferent generation rates. This proceure is certainly approximate, in that it assumes a uniform efect ensity across the cell at all stages of light-soaking; we have not explore the error associate with this approximation. Experimentally, V OC only eclines about 0.04 V (to the value enote measure saturation ) when G = cm -3 s -1. As inicate in Fig. 2, a ecline by 0.04 V implies that most of the photocarrier recombination traffic still flows through the bantails even in the saturate, light-soake state. For the four intensities, the agreement between the experimental measurements an the calculations is fairly goo through the initial stages of the ecay of V OC. For longer times the measure values for V OC fall much more slowly than preicte by eq. (6). Experimentally, efect ensities also saturate for sufficiently extene light-soaking. The saturation effect is usually attribute to a combination of thermal an light-inuce annealing of efects [2,6,11]; we have not inclue the annealing processes in the present work DISCUSSION The H-collision moel, in conjunction with the bantail+efect recombination moel, preicts eviations from the t 1/3 form for the epenence of the efect ensity when the light-soaking time excees the time require for crossover from bantail recombination to efect recombination. In Fig. 3 we show the preictions for the efect ensity obtaine from numerical integration of eq. (6), an using the highest photogeneration rate (2x10 21 cm -3 s -1 ) of our experiments. The calculations show the expecte t 1/3 form for the earlier times. There is a softening that occurs aroun 10 hours, which correspons to the change in the recombination mechanism. For lower efect ensity, the majority of the recombination traffic goes through the bantail. For larger ensities, the majority goes through efects. A t 1/5 form is a goo fit at longer times, as can be shown using eq. (6) an the Defect Density (cm -3 ) t 1/ Light-Soaking Time (hours) Fig. 3: Calculate growth of the efect ensity uring light-soaking (photogeneration rate cm -3 s -1 ); the calculation was base on the hyrogen-collision moel of efect generation (C SW = 10 cm -3 s) an the bantail+efect recombination moel. The calculation shows a cross-over from t 1/3 to t 1/5 kinetics when recombination traffic switches from the bantails to the efects. t 1/5 G = 2x10 21 cm -3 s -1

7 A assumption Rt 1 N. Experimentally, after about 10 hours V OC begins to show saturation behavior (cf. Fig. 1), which we attribute to light-inuce annealing. At least for the present samples, it appears that the long-time saturation woul obscure irect observation of the t 1/5 regime. It is an interesting coincience that two apparently inepenent time scales must be fairly similar to account for the measurements in Fig. 1. The two times are (i) the crossover time at which recombination traffic switches between the bantail an efects, an (ii) the lightinuce annealing time. This apparent coincience is the reason offere by the present viewpoint for the fact that open-circuit voltages ecline only moestly uring light soaking (roughly by the thermal voltage k B T/e = V), as well as for the fact that most irect experiments on the kinetics of the efect ensity are reasonably consistent with the t 1/3 form. Of course, it is unlikely that two entirely inepenent processes woul appear on the same timescale, but we o not presently have a satisfactory explanation for it. This work has been supporte through the Thin Film Photovoltaics Partnership of the National Renewable Energy Laboratory (NDJ at Syracuse an ZDJ at Unite Solar). REFERENCES 1. J. Liang, E. A. Schiff, S. Guha, B. Yan, an J. Yang, in P. C. Taylor, et al., e. Amorphous an Nanocrystalline Silicon Technology 2005 (Mater. Res. Soc., Symp. Proc. Vol. 862, Pittsburgh, 2005), in press. 2. H. M. Branz, Phys. Rev. B l59, 5498 (1999). 3. M. Stutzmann, W. B. Jackson, an C. C. Tsai, Phys. Rev. B 32, 23 (1985). We note that the value for C SW that we extract from this paper is much larger than the values obtaine from later, but otherwise similar, experiments [5,6]. 4. J. Yang, A. Banerjee, an S. Guha, Appl. Phys. Lett. 70, 2975 (1997). 5. H. Park, J. Liu, an S. Wagner, Appl. Phys. Lett. 55, 2658 (1989). 6. T. Unol, J. Hautala, an J. D. Cohen, Phys. Rev. B 50, (1994). 7. P. Strains, unpublishe. 8. P. Tzanetakis, N. Kopiakis, M. Anrouliaki, C. Kalpouzos, P. Strains, an H. Fritzsche, J. Non-Cryst. Solis , (1996). 9. P. Strains an H. Fritzsche, J. Non-Cryst. Solis 200, 432 (1996). 10. See the recent review of H. Fritzsche, Annu. Rev. Mater. Res , H. Gleskova, J. N. Bullock, an S. Wagner, J. Non-Cryst. Solis , 183 (1993).