MG DOPING AND ALLOYING IN ZN3P2 HETEROJUNCTION SOLAR CELLS

Transcription

1 MG DOPING AND ALLOYING IN ZN3P2 HTROJUNCTION SOLAR CLLS Gregory M. Kimball, Nathan S. Lewis, Harry A. Atwater California Institute of Tehnology, Pasadena, California, United States ABSTRACT Zin phosphide (Zn3P2) is a promising and earthabundant alternative to traditional materials (e.g. CdTe, CIGS, asi) for thin film photovoltais. We report the fabriation of Mg/Zn3P2 Shottky diodes with Voe values reahing 55 mv, Jse values up to 21.8 malm 2, and photovoltai effiieny reahing 4.5%. Previous authors have suggested that Mg impurities behave as ntype dopants in Zn3P2, but ombined Hall effet measurements and Seondary Ion Mass Spetrometry (SIMS) show that 1 17 to 1 19 m 3 Mg impurities ompensate ptype doping to form highly resistive Zn3P2. Further devie work with modified ITO/MglZn3P2 heterojuntions suggests that the ITO apping layer improves a passivation reation between Mg and Zn3P2 to yield high voltages > 5 mv without degradation in the blue response of the solar ell. These results indiate that at least 81% effiieny ell is realizable by the optimization of Mg treatment in Zn3P2 solar ells. INTRODUCTION Zin phosphide (Zn3P2) has signifiant potential as an absorber in thin film photovoltais, with a reported diret 1.5 ev band gap, high (> 14_1 5 m 1 ) light absorbane in the visible region [1], and long (51 IJm) minorityarrier diffusion lengths [2). To date, Zn3P2 has been produed almost exlusively with ptype doping [3], preventing the fabriation of pn homojuntions. Solar ells using Zn3P2 have therefore been onstruted from Shottky ontats, pn semiondutor heterojuntions [4], or liquid ontats [5], with Mg/Zn3P2 Shottky diodes having exhibited >6% solar energyonversion effiieny [6). Preliminary evidene from early work with MglZn3P2 Shottky diode devies suggests that annealing leads to formation of a p n homojuntion [78] with Mg interstitials behaving as n type dopants. In this manusript, the eletroni and material properties of Mg impurities in Zn3P2 are studied by the Hall effet and Seondary Ion Mass Spetrometry (SIMS). Investigation of Ag and P dopants is also inluded in the Hall effet experiments. The interfaial reation between Mg and Zn3P2 is studied by SIMS and impliated in improved juntion quality in solar ells with a MglZn3P2 interfae. The fabriation and photovoltai properties of both Mg/Zn3P2 Shottky diodes and modified ITO/Mg/Zn3P2 heterojuntion solar ells are presented. XPRIMNTAL The Zn3P2 samples used in this study were grown by a physial vapor transport proess (Fig. 1). Red phosphorus hips and zin shot ( %, Alfa Aesar) were ombined at 85 C to form Zn3P2 powders. Using proedures desribed previously [914] the powders were then grown into polyrystalline boules 1 m in diameter and 4 m in length, with grain sizes of 15 mm 2. The resulting rystals were died with a diamond saw and had asgrown resistivity between 12 n m. Annealing with white phosphorus in sealed ampoules at 4 C for 2 hours was effetive at reduing the wafer resistivity to 2 n em due to doping by phosphorus interstitials [1). Both undoped and Pdoped samples were polished with diamond paste to produe Zn3P2 wafers. Samples with 1 em diameter and 56 IJm thikness were ethed for 3 s in 23% (v:v) Br2 in CH3H, rinsed in CH3H, dried under a stream of N2, and used promptly thereafter. Mg/Zn3P2 Shottky diodes were fabriated from both undoped and Pdoped Zn3P2 samples using proedures similar to those previously reported [15). Mg films of thikness 2 nm were deposited by RF magnetron sputtering on freshly ethed Zn3P2 wafers without any sputter ething of the sample. Bak ontats of Ag with 2 nm thikness were then deposited by vauum evaporation. Using optial photolithography a series of devies with.25 mm 2 ative area and arrays of 2 IJm bus bars were patterned by ething through the Mg top ontat with an aqueous solution of 75 mm disodium DT A and 3% (v:v) H22 adjusted to ph 1. Modified ITO/MglZn3P2 heterojuntion solar ells were also fabriated from Pdoped Zn3P2 samples. Freshl ethed wafers were patterned with an array of 1 mm ative area devies by optial photolithography. Mg films of thikness 3 nm followed by ITO films of thikness 1 nm were deposited by RF magnetron sputtering on the patterned Zn3P2 wafers without any sputter ething of the sample. Bak ontats of Ag with 1 nm thikness were also deposited by RF magnetron sputtering. The modified heterojuntion ells were subjeted to mild air annealing at 1 C. Photovoltai performane of both types of devies was studied without additional anti refletive oatings under 1. sun AM 1.5G illumination at room temperature. xpliit Ag and Mgdoping of Zn3P2 samples was performed by solid state diffusion. thed Zn3P2 wafers were metalized with Ag or Mg of thikness s 8 nm and subjeted to heat treatment at temperatures in the range 139

2 1 em diameter Figure 1 Photographs of Zn3P2 as the solar ell fabriation proess proeeds: (lokwise from top left) high purity powder, a large rystal, a died wafer, and a PZn3P2/Mg Shottky diode solar ell. of 14 C for up to 24 hours, followed by hemial removal of the remaining metal dopant soure. Samples were typially annealed under passive vauum of 15 torr with 15 mg of added red phosphorus, and in some ases an additional 4 mg of Mg was added. Samples intended to be uniformly doped for Hall measurements were annealed at 4 C for 24 hrs, ethed with 2% Br2 in CH3H, and ontated at the orners with indium. Samples intended to be Mgdoped for SIMS analysis were annealed at 13 C under ative vauum of 1 6 torr for 21 min and stripped of exess Mg with aqueous H22/DTA. After the doping treatment, the Zn, P, and Mg onentration profiles were analyzed by SIMS. To alibrate the ount rate for Mg in Zn3P2, a standard was fabriated by ionimplantation with m 2 Mg ions. An primary ion beam at 1 kv and 2 na was seleted to ahieve both a high ount rate for Mg+ ions and good depth resolution. RSULTS Both undoped and Pdoped Zn3P2 wafers show signifiant photovoltai response in MgShottky diodes as illustrated in Fig. 2. Cells made from undoped Zn3P2 wafers typially exhibited effiienies of 1.1.5%, with Voe reahing 55 my; a large series resistane prevented good urrent olletion and high fill fators. The large bulk resistivity of the 56 IJm thik, undoped wafer is the primary ontributor to the 11 ko series resistane. Cells made from Pdoped Zn3P2 typially have effiieny between %, with somewhat redued Voe values of 4 my, and better overall performane due to improved urrent olletion and fill fators. The solar ell parameters of representative devies are given in Table 1. xtensive Hall measurements were onduted on Zn3P2 samples doped with Ag, P and Mg and in all ases ptype ondutivity was observed (Table 2). The eletroni om 32 8 "' 28 :DIoI't1lDO.. 7 () Phosphorusdoped 24 6.s :e: 'iii / C 16 4 Undoped C 12 3 u u Voltage (V) Figure 2 JV harateristis of several Mg/Zn3P2 Shottky diode devies under simulated AM1.5 illumination fabriated from asgrown (undoped) and white phosphorus annealed (Phosphorusdoped) Zn3P2 wafers. Jse (ma m 2 ) Voe (V) FF T] (%) Undoped Undoped Undoped Pdoped Pdoped Pdoped Table 1 Photovoltai parameters of MgZn3P2 Shottky diode devies fabriated from asgrown (undoped) and white phosphorus annealed (Pdoped) Zn3P2 wafers. properties of Agdoped Zn3P2 samples annealed in white phosphorus are broadly onsistent with previous reports and ptype dopinp in the high 1 1 m 3 ith hole m? bility of 17 ± 1 m 2 V S 1 an be readily ahieved. Additional Ag diffusion soure does not appear to inrease the aeptor onentration above the threshold of 8 x 1 17 m 3. The measurements of Pdoped Zn3P2 samples are onsistent with previous reports, orresponding to aeptor onentrations in the mid 1 16 m 3 and hole mobility in the range of 1113 m 2 V 1 S 1. Substrates doped with Mg at 4 C show ptype ondutivity with very low aeptor onentrations in the range of to 1 1'1 m 3 despite Mg onentrations estimated in the range of 1 17 to 1 19 m 3. 14

3 SIMS profiling of the Mg ontent in Zn3P2 samples doped by solid soure diffusion revealed a narrow region of reation at the MglZn3P2 interfae as well as penetration of Mg into the substrate at high onentration (Fig. 3a). For diffusions arried out at 1 C for 1 minutes, the first 15 nm of the sample shows greatly inreased Mg and P ounts along with extensive Zn depletion. Crystal orientation dependene was observed for the Mg profile at depths greater than 1 nm with peak onentrations in the range of 1 18 to 1 2 m 3 (Fig. 3a (i), (ii» that varied between rystal domains. At a depth S 5 nm, samples treated at 1 C show a return to bakground levels in Mg onentration, < 1 15 m 3. On the other hand, samples treated at 3 C for 1 minutes show an extensive reation at the MglZn3P2 interfae in addition to high levels of Mg inorporation in the Zn3P2 substrate. The first 5 nm of the sample shows strongly elevated P and Mg ounts as well as greatly redued Zn ounts onsistent with a MgxZn3 xp2 alloy rather than a pure Zn3P2 matrix. At depths approahing 8 nm to 1 IJm the data is onsistent with a Zn3P2 matrix doped 1 19 to 1 2 m 3 with Mg. In samples proessed at 3 C the depth at whih Mg onentration reahed baseline was not reahed but appears to be greater than 2 IJm. Dopant Anneal p (m 3 ) IJp deposition Ambient (m 2 Ns) Ag (8 nm) P4 7.5 ±.4 x ± 1 Ag (12 nm) P4 7.6 ±.8 x ± 2 Ag (1.3 nm) P4 3. ±.1 x ± 1 none P4 3.7 ±.4 x ± 1 Mg (8 nm) P4 3.8 ±.8 x ± 3 Mg (8 nm) P4 + Mg 5.4 ±.8 x ± 18 Mg (8 nm) vauum 6. ± 3.2 x ±25 Mg (8 nm) Mg 1.5±.4x ±2 Table 2 Carrier onentration and mobility results from Hall measurements of doped Zn3P2 samples. Ag and Mg dopants were introdued by solid soure diffusion at 4 C for 24 hours in vauum ampoules ontaining small quantities of P4 and/or Mg. The samples tested exlusively exhibit ptype doping. (a) "I.t (,) ' 1 2 ' 1 " 1 '7 "I.[ 122 C : (,) ' 1 C, 1 min p zn 1 ' Depth (nm) o Depth (nm) Figure 3 SIMS profiles of Zn, P and Mg atomi onentrations as a funtion of depth in Mgdiffused Zn3P2 samples. The profiles were olleted from samples annealed for (a) 1 C and (b) 3 C under ative vauum of 1 6 torr for 1 min. Traes (i) Mg and (ii) Mg orrespond to the profiles of two different regions of the sample. Pdoped Zn3P2 wafers were also used to fabriate solar ells based on a modified ITO/MglZn3P2 heterojuntion devie geometry. Although asfabriated ells exhibit poor performane and Voe S 1 mv, mild air annealing at 1 C for 1 min greatly improves the juntion properties with Voe reahing 54 mv (Fig. 4a). The overall low external quantum effiieny throughout the visible is onsistent with expeted refletion losses from the optially thik 3 nm Mg layer (Fig. 4b). Heat treatments of longer than 16 hrs were found to affet a signifiant inrease in the olletion effiieny. Annealed samples also exhibited greatly inreased series resistane, typial of the Mgdoped samples used for Hall measurements /1/$ I 141

4 (a) 4 11 e, air anneal 1 asfabriated "' u 3 2 min +1 min.?: (b) w ::> f;l.2 ::> a rn.1 x w..7 hrs Voltage (V) Photon nergy (ev) [1 e, air anneaij asfabriated 2min +1 min Ll' ' 1' _od...j Wavelength (nm) C 1 Figure 4 Photovoltai performane observed in modified ITO/MglZn3P2 heterojuntion solar ells as a funtion of air annealing times inluding (a) light JV traes under simulated AM1.5 illumination and (b) Q harateristis. DISCUSSION The Mg/Zn3P2 Shottky devies reported here show some improvements over reord ells of similar design [6]. In undoped Zn3P2 devies we observe Voe values reahing 55 mv, somewhat higher than the typial Voe of 4 mv reported in devies without heat treatment. In Pdoped Zn3P2 devies we observe Jse inreases of 3% relative to previous reord ells, an effet that is attributable to the high minority arrier diffusion lengths <: 7 IJm of the Zn3P2 substrates used for the fabriation [16]. Continuing work to ombine the high voltage of the ells based on undoped Zn3P2 with the high urrent of the ells based on Pdoped Zn3P2 is expeted to yield solar ells with solar energy onversion effiieny > 8%. In previous literature, MglZn3P2 Shottky devies subjeted to heat treatments were observed to have inreased juntion depth and higher Voe onsistent with the transition from a Shottky diode to a diffused pn homojuntion [78]. Mg dopants forming ntype Zn3P2 was suspeted to reate the Zn3P2 pn homojuntion, but the results in Table 2 suggest that Mg impurities behave as traps rather than wellionized donors and form highly ompensated ptype material. For samples annealed at 1 C for 1 min, the Mg diffusion depth determined by SIMS, 35 nm (Fig. 3a), is remarkably shorter than the juntion depth of 1.3 IJm estimated by Shushan et al for similarlytreated samples [7], providing further indiret evidene that Mg do not behave as ntype dopants. SIMS analysis of Mgdiffused Zn3P2 is onsistent with the formation of MgxZn3 xp2 at the interfae, distint from the Mg3P2 that as has been observed using sputter ASIXPS by other authors [17]. The reated layer appears to be important for improving the juntion properties of the MglZn3P2 interfae (Figure 4a) by ating as a passivation or holebloking layer. At the same time, Hall measurements on Mgdoped Zn3P2 suggest that Mg impurities in bulk Zn3P2 degrade devie performane. As a result the optimal annealing onditions for Mg/Zn3P2 solar ells minimize Mg diffusion while still allowing MgxZn3 xp2 formation. The modified ITO/MglZn3P2 heterojuntion devies reported here demonstrate the improvements in photovoltai performane that result from the interfaial reation between Mg and Zn3P2. The very low barrier height of the solar ells asfabriated is likely due to surfae defets and oxidation aused by the photolithographi patterning proess. Heating the devie in air at 1 C for 1 min greatly improves the barrier height, and the SIMS profiles indiate that the benefit omes from the formation of MgxZn3 xp2. Although the reported ells appear to be absorptionlimited due to the Mg film, prolonged heating redues refletion losses by diffusing exess Mg into the substrate. An additional effet of the heat treatment is a large inrease series resistane attributable to the ompensating effet of Mg impurities in Zn3P2. In ontrast to devies with the thin Mg film exposed to air [7], the ITOapped devies presented here show substantial blue response even after heating for several days. The ITO apping layer is suspeted to moderate the reation between Mg and Zn3P2, resulting in an interfae with low surfae reombination veloity. CONCLUSIONS We report improvements over previous results in photovoltai performane for solar ells based on p Mg/Zn3P2 Shottky diodes. In undoped Zn3P2 devies we observe Voe values up to 55 mv, and in Pdoped Zn3P2 devies we observe Jse inreases of 3% relative to previous reord ells. We have also begun the first analysis of Mg dopant profiles in Zn3P2 substrates by ombined Hall measurements and SIMS profiling to determine that Mg dopants do not behave as wellionized ntype dopants. Further devie work with modified ITO/MglZn3P2 heterojuntions suggests that the TCO /11$ I 142

5 apping layer improves a passivation reation between Mg and Zn3P2 to yield high voltages reahing 54 mv without reduing the urrent olletion of the solar ell. These results indiate that at least 81% effiieny ell is realizable by the optimization of Mg treatment in Zn3P2 solar ells. ACKNOWLDGMNT We aknowledge Yunbin Guan and the Division of Geologial and Planetary Sienes at Calteh for assistane olleting SIMS data. This work was supported by the Offie of nergy ffiieny and Renewable nergy, US Department of nergy under grant DFG368G186, the Calteh Center for Sustainable nergy Researh (CCSR), as well as a partnership with the Dow Chemial Company. One of us (GMK) aknowledges support under an NDSG graduate fellowship. RFRNCS Transport Method", Ata Physia Polonia A 69 (6), 1986, pp [13] F. C. Wang, R. H. Bube, R. S. Feigelson and R. K. Route, "SingleCrystal Growth of Zn3P2", Journal of Crystal Growth 55 (2), 1981, pp [14] G. M. Kimball, presented at the Thirtythird I Photovoltai Speialists Conferene, San Diego, CA, USA, 28. [15] A. Catalano, J. V. Masi and N. C. Wyeth, presented at the Proeedings, 2nd. C. Photovoltai Solar nergy Conferene, Berlin, [16] G. M. Kimball, A. M. Muller, N. S. Lewis and H. A. Atwater, "PhotolumineseneBased Measurements of the nergy Gap and Diffusion Length of Zn3P2", Applied Physis Letters 95 (11), 29, pp. 3. [17] L. L. Kazmerski and P. J. Ireland, "Surfae and Interfae Properties of Zn3P2 SolarCells", Journal of Vauum Siene & Tehnology 18 (2), 1981, pp [1]. A. Fagen, "OptialProperties of Zn3P2", Journal of Applied Physis 5 (1), 1979, pp [2] N. C. Wyeth and A. Catalano, "Spetral Response Measurements of MinorityCarrier Diffusion Length in Zn3P2", Journal of Applied Physis 5 (3), 1979, pp [3] A. Catalano and R. B. Hall, "Defet Dominated Condutivity in Zn3P2", Journal of Physis and Chemistry of Solids 41 (6), 198, pp [4] F. C. Wang, A. L. Fahrenbruh and R. H. Bube, "Transport Mehanisms for MglZn3P2 Juntions", Journal of Applied Physis 53 (12), 1982, pp [5] M. Bhushan, J. A. Turner and B. A. Parkinson, "Photoeletrohemial Investigation of Zn3P2", Journal of the letrohemial Soiety 133 (3), 1986, pp [6] M. Bhushan and A. Catalano, "Polyrystalline Zn3P2 ShottkyBarrier SolarCells", Applied Physis Letters 38 (1), 1981, pp [7] M. Bhushan, "Mg Diffused Zin Phosphide NIP Juntions", Journal of Applied Physis 53 (1), 1982, pp [8] A. Catalano and M. Bhushan, "videne of PN Homojuntion Formation in Zn3P2", Applied Physis Letters 37 (6), 198, pp [9] A. Catalano, "The Growth of Large Zn3P2 Crystals by Vapor Transport", Journal of Crystal Growth 49 (4), 198, pp [1] S. Fuke, "Growth and Charaterization of Zin Phosphide Crystals", Journal of Crystal Growth 87 (4), 1988, pp [11] A. Kuroyanagi, "SingleCrystal Growth and Charaterization of Zin Phosphide", Journal of Crystal Growth 1 (12), 199, pp. 14. [12] J. Misiewiz, F. Kroliki, M. Lewiki and J. F. Kasprzak, "Growth of Zn3P2 Crystals by Gas 143