We present temperature-dependent measurements and modeling for a thickness series of hydrogenated amorphous silicon nip solar cells. The comparison indicates that the maximum power density (PMAX) from the as-deposited cells has achieved the hole-mobility limit established by valence bandtail trapping, and PMAX is thus not significantly limited by intrinsic-layer dangling bonds or by the doped layers and interfaces. Measurements of the temperature-dependent properties of light-soaked cells show that the properties of as-deposited and light-soaked cells converge below 250 K; a model perturbing the valence band tail traps with a density of dangling bonds accounts adequately for the convergence effect

Guha, S.

Liang, Jiang

Schiff, Eric A

Yan, Baojie

Yang, Jeff

English

Syracuse University Research Facility and Collaborative Environment

Syracuse University SURFACE Physics College of Arts and Sciences 2006 Hole Mobility Limit of Amorphous Silicon Solar Cells Jiang Liang Syracuse University Eric A. Schiff Syracuse University S. Guha United Solar Ovonic Corporation Baojie Yan United Solar Ovonic Corporation Jeff Yang United Solar Ovonic Corporation Follow this and additional works at: https://surface.syr.edu/phy  Part of the Physics Commons Recommended Citation "Hole mobility limit of amorphous silicon solar cells," Jianjun Liang, E. A. Schiff, S. Guha, Baojie Yan, and J. Yang, Appl. Phys. Lett. 88 063512-063514 (2006). This Article is brought to you for free and open access by the College of Arts and Sciences at SURFACE. It has been accepted for inclusion in Physics by an authorized administrator of SURFACE. For more information, please contact surface@syr.edu. Hole-mobility limit of amorphous silicon solar cellsJianjun Lianga and E. A. SchiffDepartment of Physics, Syracuse University, Syracuse, New York 13244-1130S. Guha, Baojie Yan, and J. YangUnited Solar Ovonic Corporation, Troy, Michigan 48084Received 1 August 2005; accepted 6 December 2005; published online 10 February 2006We present temperature-dependent measurements and modeling for a thickness series ofhydrogenated amorphous silicon nip solar cells. The comparison indicates that the maximum powerdensity PMAX from the as-deposited cells has achieved the hole-mobility limit established byvalence bandtail trapping, and PMAX is thus not significantly limited by intrinsic-layer danglingbonds or by the doped layers and interfaces. Measurements of the temperature-dependent propertiesof light-soaked cells show that the properties of as-deposited and light-soaked cells converge below250 K; a model perturbing the valence band tail traps with a density of dangling bonds accountsadequately for the convergence effect. © 2006 American Institute of Physics.DOI: 10.1063/1.2170405Solar cells based on hydrogenated amorphous silicona-Si:H and related materials are now manufactured in largequantities, so it is remarkable that there is no consensus re-garding the optoelectronic processes that determine theirphotovoltaic conversion efficiency. There are at least twobroad issues that have led to ambiguities regarding the de-vice physics. First, the solar cells are nip or pin structures,and the cell efficiency is potentially affected by the proper-ties of the intrinsic and doped layers and also by the inter-faces between them. Second, at least for the intrinsic layer,both disorder limitation of carrier mobilities, and also carriercapture by deep levels such as dangling bonds, are eachknown to be important, but it is difficult to distinguish thetwo effects on the cell’s efficiency.In this letter, we present temperature-dependent mea-surements and analyses for a series of a-Si:H nip solar cellsprepared at United Solar Ovonic Corporation that indicatethat the efficiencies of these cells at solar intensities aremostly determined by the hole mobility of the intrinsic layer.Essentially the same conclusion of mobility or space-changelimited photocurrent was reached recently for polymer-fullerene blend solar cells.1 Temperature-dependent measure-ments are useful for exploring hole mobility effects in a-Si:Hsolar cells because the strong temperature dependence of thehole drift mobility in a-Si:H gives a fairly distinctive tem-perature dependence for the cell efficiency. The statementthat the efficiencies are near the hole mobility limit impliesthat the density of deep levels is sufficiently low that it has aminor effect on the efficiency, and that the p / i and n / i inter-faces are sufficiently good that they are behaving nearly ide-ally. We also present measurements for the light-soakedstate, where deep levels do have a clearly measurable effect;the mobility-limitation idea remains roughly applicable. Weconclude that improved efficiencies of a-Si:H solar cells willlikely require further improvements in the hole drift mobility.For these experiments, six a-Si:H nip solar cells weredeposited using rf glow discharge on stainless steel sub-strates. The n and p layers were the same in all depositions;the deposition time for the intrinsic layer was chosen to giveintrinsic layer thicknesses from 185 to 893 nm. The cellswere not optimized for the best efficiency, and in particulardo not have a highly reflecting back contact. Special atten-tion was paid to the hydrogen dilution in the intrinsic layer tomaintain a good material quality throughout the entire intrin-sic layer and to avoid nanocrystallite inclusion. Details of thedeposition procedures have been given elsewhere.2 As-deposited properties of the cells were measured under a solarsimulator; for the thickest intrinsic layer 893 nm a typicalcell had an open-circuit voltage VOC of 0.982 V, a shortcircuit current density JSC of 14.4 mA/cm2, a fill factorFF of 0.559, and a maximum power density PMAX of7.9 mW/cm2.Further studies were done using a 30 mW, near-infrared685 nm wavelength laser. We chose to use this laser be-cause its wavelength is absorbed fairly uniformly throughoutthe intrinsic layers of the cells, which simplifies modeling ofthe measurements. In Fig. 1 we present the temperature de-pendent normalized maximum power density PMAXT /JsTand VOCT for four cells with differing intrinsic-layer thick-ness. Js is the photocurrent density measured at −2 V, whichis a good approximation to the saturated reverse-bias photo-current over most of the temperature range. The measure-ments were done at constant laser flux. The average photo-generation rate at 295 K was G=3.31020 cm−3 s−1; G doesvary with T due to the temperature-dependent band gap ofa-Si:H.As has been reported by most previous workers, VOC isnearly thickness independent. PMAX/Js declines markedlywith thickness, which indicates that photocarriers generateddeep inside a thick cell are likely to recombine instead ofcontributing to power generation. The solid lines in Fig. 1represent computer calculations AMPS-1D code3 using whatwe consider to be the simplest reasonable model for ana-Si:H nip solar cell. The crucial electrical parameters de-scribe hole transport in the intrinsic layer. The code uses theexponential valence bandtail trapping model4,5 to describehole transport. The parameters we used are given in Table I;as described elsewhere,6 they have been chosen for consis-tency with hole drift mobility “time-of-flight”measurements.7 The electron parameters are given in Table IaElectronic mail: jiliang@syr.eduAPPLIED PHYSICS LETTERS 88, 063512 20060003-6951/2006/886/063512/3/$23.00 © 2006 American Institute of Physics88, 063512-1for completeness, but the calculations are fairly insensitive tothem. This is expected because electron drift-mobilities arehundreds of times larger than hole drift mobilities ina-Si:H.5,8 Deep levels were not included in the calculationsof Fig. 1. We used idealized p and n layer parameters forwhich the precise values have little effect on calculated cellproperties; prior experiments indicate that doped layers andinterfaces are not limiting VOC in contemporary cells.9 Thetemperature-dependent photogeneration rate GT was deter-mined from the measured, temperature-dependent photocur-rent under reverse bias for the thinnest cell 186 nm. Thetemperature-dependent bandgap EGT was determined frommeasurements of the electroabsorption spectrum that are notshown here; this method10 gave results consistent with earlierreports.11As can be seen from the figure, these calculations give agood account for the temperature-dependent magnitudes ofVOC and PMAX/Js. The near thickness independence of VOCand its increase with decreasing temperature may be roughlyunderstood using the model that equates eVOC e is the elec-tronic charge to the separation of electron and hole quasi-Fermi levels in the intrinsic layer.4,8 For the thicker samples,the decline in PMAX/Js with decreasing temperature is a con-sequence of the rapid decline in the hole drift mobility withtemperature in a-Si:H.5 As the hole drift mobility declines,the region from which holes can be collected shrinks, andleads to reduced power from a thick cell.8 At the highesttemperatures, there is evidently a discrepancy between themodel and the PMAX/Js measurements; we believe that thisreflects the onset of the effects of deep levels danglingbonds.Modelers generally include deep levels when they studya-Si:H solar cells,12,13 but we do not believe that deep levelsdominate the values of PMAX and VOC of the as-depositedcells studied here. One reason for our view is that the mea-sured drift mobilities of holes in a-Si:H essentially establishthe largest values of PMAX that can be obtained from thecells, and Fig. 1 indicates that the actual power from cells isclose to these maximum values. If the densities of deep lev-els were large enough, they would lower PMAX and VOC no-ticeably below the calculated values of Fig. 1. A second rea-son for believing that a density of deep levels is notdominating VOC and PMAX is illustrated in Fig. 2, where thedashed line is a calculation of PMAX/Js based on the assump-tion that deep levels do dominate hole trapping. PMAX/Jscalculated using deep levels decreases as the temperature Tincreases; both the measured PMAX/Js and the calculationbased on hole mobilities increase with T.For the deep level calculation in Fig. 2, we set the band-tail trap densities to zero. We used a donor-type 0/ +  deeplevel that was electrically neutral in the dark. The coeffi-FIG. 1. Temperature-dependent measurements of VOC and the PMAX/Js forfour as-deposited a-Si:H nip solar cells with varying intrinsic layer thicknessare indicated by the symbols. Measurements were done at constant laserflux; the average photogeneration rate G at 295 K was 3.3x1020 cm−3 s−1.The lines indicate corresponding calculations for a model based primarily onhole drift-mobility measurements.TABLE I. a-Si:H solar cell modeling parameters see Ref. 6, 8, and 11. Solar cell performance is mainlysensitive to the first 5 parameters, governing the hole drift mobility and the bandgap.p 0.3 cm2/V s Band mobility of holesEv 0.04 eV Width of exponential valence band tailNv 41020 cm−3 Valence band effective density-of-statesbvp 1.310−9 cm3 s−1 Trapping coefficient h+→V0 valence band tailEg 1.74 eV Electrical band gapdEg /dT −4.710−4 eV/Kbvn 1.010−9 cm3 s−1 Trapping coefficient e−→V+ valence band tailn 2 cm2/V s Band mobility of electronsEc 0.02 eV Width of exponential conduction band tailNc 41020 cm−3 Conduction band effective density-of-statesbcn 1.310−9 cm3 s−1 Trapping coefficient e−→C0 conduction band tailFIG. 2. Temperature-dependent calculations for PMAX/Js of an a-Si:H nipsolar cell based on a deep-level model are shown as the dashed line; theintrinsic layer thickness is 893 nm. The corresponding experimental mea-surements symbols and valence band tail based calculations from Fig. 1are also shown for reference.063512-2 Liang et al. Appl. Phys. Lett. 88, 063512 2006cients for hole and electron capture by the deep level chosento be bdp=7.510−9 and bdn=410−8 cm3 s−1, respectively.Given p=0.3 cm2/V s, they are consistent with hole deep-trapping experiments.14 We neglected the slight temperaturedependences. The defect density Nd is 5.71016 cm−3 forFig. 2, which fits the measured power at 230 K.While deep levels do not seem to have a noticeable ef-fect on the as-deposited state of these cells, a growth in deep-level density during illumination is the most plausiblemechanism for the degradation of a-Si:H based solar cellsduring light soaking.15 In Fig. 3 we present the measure-ments for VOC and PMAX/Js for an 893 nm cell before andafter 200 h of light soaking at 295 KG=3.31020 cm−3 s−1. The PMAX measured at 295 K degradedabout 30%, which is comparable to the saturated degradationof thick cells after long-term exposure to solarillumination.16 It is interesting that there is little difference inthe light-soaked and the as-deposited states when measuredat 230 K. This convergence of the two states at lower tem-peratures has not been noted in previous solar cell studies.We were able to account for the convergence effect usingthe modeling procedures described earlier. For the light-soaked state, we added a density of deep levels to the bandtail traps that were used to calculate the properties of theas-deposited state.17 The deep level trapping coefficients bdpand bdn were noted earlier; the density of defects ND wasadjusted to 21016 cm−3 in order to fit the measured valueof VOC at 295 K after light soaking. This calculation gives asatisfactory account for the magnitude of PMAX/Js no pa-rameters were further adjusted, as well as for thetemperature-dependences of VOC and of PMAX/Js. The mea-surements of VOC at lower temperatures are somewhatsmaller than the calculated values. We speculate that thiseffect is due to a nonideal p layer.Heuristically, the convergence effect may be understoodfrom the splitting of the electron and hole quasi-Fermi levelsin the intrinsic layer. This splitting increases as T falls asevidenced by VOCT. While the density of deep levels isconstant, the density of bandtail states between the twoquasi-Fermi levels increases with this splitting—thus ac-counting qualitatively for the increased importance of band-tails at lower T.The research has been supported by the National Renew-able Energy Laboratory through the Thin Film PhotovoltaicsPartnership subcontract Nos. NDJ-2-30630-24 and ZDJ-2-30630-19.1V. D. Mihailetchi, J. Wildeman, and P. W. M. Blom, Phys. Rev. Lett. 94,126602 2005.2J. Yang, A. Banerjee, and S. Guha, Appl. Phys. Lett. 70, 2975 1997.3H. Zhu, A. K. Kalkan, J. Hou, and S. J. Fonash, AIP Conf. Proc. 462, 3091999.4T. Tiedje, Appl. Phys. Lett. 40, 627 1982.5Q. Gu, Q. Wang, E. A. Schiff, Y.-M. Li, and C. T. Malone, J. Appl. Phys.76, 2310 1994, and references therein.6K. Zhu, J. Yang, W. Wang, E. A. Schiff, J. Liang, and S. Guha, Mater. Res.Soc. Symp. Proc. 762, 297 2003.7Actual calculations require the densities of valence and conduction bandtail traps gv0=61021 and gc0=1.61022 cm−3 s−1, respectively. These arenot independent parameters since we use the assumption that the bandedgelies within the band tail see Refs. 6 and 8.8E. A. Schiff, Sol. Energy Mater. Sol. Cells 78, 567 2003.9J. M. Pearce, R. J. Koval, A. S. Ferlauto, R. W. Collins, C. R. Wronski, J.Yang, and S. Guha, Appl. Phys. Lett. 77, 19 2000.10J.-H. Lyou, E. A. Schiff, S. Guha, and J. Yang, Appl. Phys. Lett. 78, 19242001.11G. D. Cody, T. Tiedje, B. Abeles, B. Brooks, and Y. Goldstein, Phys. Rev.Lett. 47, 1480 1982.12R. Schropp and M. Zeman, Amorphous and Microcrystalline Silicon SolarCells: Modeling, Materials, and Device Technology Kluwer, Boston,1998.13U. Dutta and P. Chatterjee, J. Appl. Phys. 96, 2261 2004.14R. A. Street, J. Zesch, and M. J. Thompson, Appl. Phys. Lett. 43, 6721983.15H. Fritzsche, Annual Review of Material Research Vol. 31 Annual Re-views, Palo Alto, 2001, p. 47.16S. Guha, in Technology and Applications of Amorphous Silicon, edited byR. A. Street Springer, Berlin, 1999, p. 252.17B. Yan, J. Yang, and S. Guha, Appl. Phys. Lett. 83, 782 2003.FIG. 3. Symbols indicate temperature-dependent measurements of VOC andPMAX/Js for an 893-nm-thick cell in its as-deposited and light-soaked 200 hat 295 K states. The calculated line through the as-deposited measurementsis from the hole drift-mobility based calculation of Fig. 1; for the light-soaked state, the calculation also includes a density of deep levels.063512-3 Liang et al. Appl. Phys. Lett. 88, 063512 2006

Hole Mobility Limit of Amorphous Silicon Solar Cells

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Hole Mobility Limit of Amorphous Silicon Solar Cells

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