Key Aspects of CdTe/CdS Solar Cells

Transcription

1 phys. stat. sol. (b) 229, No. 2, (2002) Key Aspects of CdTe/CdS Solar Cells K. Durose 1 ) (a), D. Boyle (a), A. Abken (a), C.J. Ottley (b), P. Nollet (c), S. Degrave (c), M. Burgelman (c), R. Wendt (d), J. Beier (d), and D. Bonnet (d) (a) Department of Physics, University of Durham, South Rd, Durham, DH1 3LE, UK (b) Dept. of Geological Sciences, University of Durham, South Rd, Durham, DH1 3LE, UK (c) Dept Electronics and Information Systems, Pietersnieuwstraat 41, B-9000 Gent, Belgium (d) ANTEC Solar GmbH, Dr. Bonnet Weg 1, D Arnstadt, Germany (Received July 27, 2001; accepted September 30, 2001) Subject classification: Mm; Jt; Ln; Gk; Jt; S8.11; S8.13 Recent developments in the following areas are briefly reviewed: a) the electrical structure of grain boundaries in CdTe absorbers, b) impurities and non-stoichiometry in CdTe solar cells and c) use of Sb 2 Te 3 in contacts to CdTe. Nominally identical solar cells fabricated using % pure CdTe feedstock from two different suppliers were compared. Differences in the photovoltaic response and absorber grain size were correlated with the purity of the feedstock, the purer material giving the higher V oc, FF and efficiency, and larger grain size. Quantum efficiency and C V measurements indicated that the performance differences are most likely to result from reduced doping at the back contact surface in the less pure sample. A quantitative SIMS study of Sb Te contacts to CdTe reveals that annealing in air at 400 C causes an influx of Sb and O into the absorber layer. Free energy calculations indicate that this is driven by the preferential reaction of O with Sb compared to CdTe oxidation. 1. Introduction Polycrystalline thin film CdTe/CdS solar cells were first demonstrated in 1972 [1] and have now reached industrial production, while lab scale cells with efficiencies of 16% have been achieved with the ubiquitous CdCl 2 processing route. (The reader is referred to reviews for further details [2, 3].) Notwithstanding these achievements, considerable opportunities remain for reducing the specific cost (cost per watt peak power) through the understanding of the role of grain boundaries, doping (both native and impurity) and electrical contacts. Here these three key aspects of CdTe/CdS solar cells are briefly reviewed. Their inter-dependency is illustrated by a study of CdTe/CdS cells made with different feed-stocks of CdTe. Additional studies of the electrical properties of grain boundaries in CdTe and reaction-diffusion in Sb Te contacts are also presented. 2. Review of Recent Developments 2.1 Grain boundaries in CdTe absorber layers Grain boundaries can influence the photovoltaic parameters V oc, J sc and FF, and the equivalent circuit parameters R series, R shunt and diode ideality factor, n. In physical terms the important effects are grain boundary recombination, the influence of barriers on current transport and grain boundary diffusion, segregation and compensation effects [4]. In 1998 Sites et al. [4] assessed the influence of grains in CdTe absorbers (E g = 1.45 ev; h = 15.8%) by com- 1 ) Corresponding author; Tel.: ; Fax: ; ken.durose@durham.ac.uk # WILEY-VCH Verlag Berlin GmbH, Berlin, /02/ $ 17.50þ.50/0

2 1056 K. Durose et al.: Key Aspects of CdTe/CdS Solar Cells parison with single crystal GaAs (E g = 1.43 ev; h = 25.7%). Of the 9.9% performance deficit, approximately three quarters was ascribed to granularity. The largest single factor was reported as grain boundary recombination (6%). Grain size has long been recognised as important and there is an extensive literature on the recrystallisation and grain growth in CdTe that sometimes accompanies CdCl 2 processing. Broadly the following trend has emerged from a rather confusing literature. CdTe having small grains (i.e. 1 mm) generally undergoes recrystallisation and grain growth during processing. This happens for samples grown by low temperature methods, such as electro-deposition, sputtering or evaporation. Where there is a preferred orientation it can be randomised during the recrystallisation. On the other hand, layers having large grains (i.e. 1 mm), such as material grown at high temperatures (e.g. close space sublimation at 500 C) generally has a grown-in grain structure that is stable to processing (the exception is in the near-interface region where small grains can exist [5]). There would appear to be limited scope for increasing efficiency by growing conventional structures with significantly larger grains than at present. It has also long been recognised that the electrical state of grain boundaries is important and that the electrical passivation of grain boundary states could bring improvements in performance. A significant early study was that by Thorpe et al. [6] in which the density of grain boundary states was measured by Werner s method of photoconductive spectroscopy. In bulk CdTe the density of states was found to be in the range N(E) = m 2 ev 1 and in p-cdte this could be reduced from to or m 2 ev 1 by Li or H 2, respectively. This passivation is unstable, however. Thorpe s work preceded the widespread use of CdCl 2 processing and the influence of Cl on the grain boundary density of states remains un-investigated. However, the evidence from J V characteristics indicates that CdCl 2 processing profoundly reduces recombination in devices. More recently Woods et al. [7, 8] has inferred the form of the grain boundary potential in CdTe in layers stripped from solar cells. This was achieved by in-plane conductivity measurements as follows: a) frequency dependent conductivity measurements indicated the near grain boundary regions to be more conductive than the bulk of the grains, b) temperature dependent conductivity measurements revealed a majority carrier barrier and fitting of the curves to a thermally assisted tunnelling model allowed the extraction of a barrier height. The inferred band diagram is shown in Fig. 1a and is discussed with reference to evidence from EBIC below. Unusual plan-view injection dependent EBIC (electron beam induced current) measurements have also been used to deduce the grain boundary potential [9, 10]. The principle used was that as the density of excited carriers approaches the equilibrium majority carrier concentration, then the photovoltaic response of the p n junction diminishes rapidly, i.e. conditions of high injection are reached. If the injection is via an electron beam then a map of the spatial location of this threshold can be made (subject to some approximations [10]). Such studies indicate high carrier concentrations near to the grain boundaries, and the band diagram inferred from this is shown in Fig. 1b. Another non-standard EBIC method, R-EBIC (remote-ebic) provides a direct method of probing fields at grain boundaries. In R-EBIC, the displacement current flowing between two Ohmic contacts placed either side of a grain boundary as a function of the electron beam position is measured. The excited minority carriers are not collected directly, instead a displacement current flows as a result of the separation of

3 phys. stat. sol. (b) 229, No. 2 (2002) 1057 Fig. 1. Models of grain boundary potentials deduced from experimental studies. a) Device CdTe, in-plane conductivity work by Woods et al. [7, 8]. b) Device CdTe, injection dependent EBIC by Galloway et al. [9, 10]. c) Bulk CdTe, R-EBIC work by Durose et al. [11] the charge at grain boundary fields. For bulk undoped CdTe [11] the sense of the current indicates grain boundaries with downward band bending as in Fig. 1c, but this effect is eliminated near to Te inclusions. In bulk Cd(S,Te) the upward band bending of Fig. 1b is deduced. From the standpoint of recombination it is important to determine whether the photo-excited minority carrier electrons reach the grain boundary states and recombine there. The grain boundary potential forms of Figs. 1a and b repel electrons and this is beneficial to solar cell operation. Whether the findings of Figs. 1a and b are compatible has not been determined. For example, the spatial resolution of EBIC may not be sufficient to resolve the features of Fig. 1a. On the other hand, another model may perhaps be devised that accounts for the conductivity measurements. The work is ongoing. 2.2 Impurities and non-stoichiometry in CdTe/CdS solar cells For reasons of cost % pure (5N) CdTe feedstock is used in the fabrication of CdTe/CdS solar cells. Alarmingly there is little reported on the influence of feedstock purity on cell performance. Doping is not achieved in any conventional manner, instead the as-deposited CdTe is rendered p-type [12] by variants of the CdCl 2 process in which the material is heated to approximately 400 C in the presence of CdCl 2. Best solar performance is considered to be achieved under conditions that encourage a Te excess. The device parameter effects of this processing have been well characterised and optimised but there is no definitive picture of the doping mechanism. Of other impurities S and Cu are the most widely studied. Grain boundary diffusion of CdS into the CdTe is the subject of a large literature. Sulphur diffusion has been implicated in interface state reduction and in doping [13]. Cu is used intentionally in contacting but can be deleterious to performance as described in the next section. A recent quantitative SIMS study [14] has revealed that CdTe/CdS solar cells fabricated from 5N CdTe contain exceptionally high levels of impurities (Na: cm 3 ; Cu: cm 3 ; O: cm 3 ; S: cm 3 ; Cl: cm 3. These levels of impurities greatly exceed those expected from 5N feedstock having a total of 10 ppm or cm 3 of impurities. A high level of compensation may therefore be expected in material with a carrier concentration in the range cm 3.

4 1058 K. Durose et al.: Key Aspects of CdTe/CdS Solar Cells 2.3 Sb 2 Te 3 based contacts to CdTe solar cells It is well known that the high electron affinity of p-cdte prevents the formation of an Ohmic contact by the direct application of a metal: no practical choice of metal having a work function of greater than 5.7 ev exists. All such contacts present a Schottky barrier to the majority carrier holes and this known fundamental problem has generated many publications and a high numbers of patents. A barrier at the back contact is generally modelled as a diode in opposition to the p n junction, and is considered to be the cause of the limitation of the forward bias current, roll-over, observed in solar cell J V characteristics at temperatures just less than 300 K [15]. A frequently used method of reducing the influence of such a barrier is by doping the CdTe surface p + in order to promote tunnelling. Use of Cu in this context is controversial high Cu concentrations diminish the cell performance [16] but cells with efficiencies in the world record bracket nevertheless have used Cu. However, etching to achieve a Te-rich surface (preferably oxide free) is almost universally adopted. (Typically this is done using a nitric-phosphoric acid etchant having an action described by Sarlund et al. [17], the so-called NP etchant.) The height of the barrier on a Te-rich surface is dependent on the means by which the Te was formed and its thickness: relatively thick Te from etching (as opposed to deposition) gives barriers as low as 0.1 ev (as determined by XPS [18, 19]). Levi et al. [20] shows that NP etching decorates the grain boundaries for some microns into the CdTe, and moreover that this is likely to affect the field at the p n junction itself. This is remarkable because the CdTe is typically 5 10 mm thick: the contact is evidently a three-dimensional structure that influences the whole cell. Direct metallisation of NP etched CdTe can yield unstable cell performance. Romeo et al. [21, 22] first proposed the use of an intermediate buffer layer and demonstrated that the compound contact Sb 2 Te 3 /NP-CdTe can yield highly efficient devices. Formation of the compound Sb 2 Te 3 as a function of sputtering temperature has been investigated systematically [23]; co-sputtering onto a low temperature substrate yields a mixed amorphous/crystalline deposit that converts to crystalline Sb 2 Te 3 upon heating. Alternatively the crystalline compound can be formed by sputtering directly at temperatures above about 200 C. Schmidt et al. [24] demonstrated the stability of the Sb 2 Te 3 CdTe couple to self-reaction and identified oxidation paths. A more challenging issue is the choice of a stable metal over-layer for the Sb 2 Te 3. Ni and V have been ruled out on account of their predicted and observed instability to reaction to yield stable tellurides [24]. (Temperature dependent free energy calculations have also been reported [23].) Boyle et al. [25] identified the possibility of using refractory metals such as Mo, Ta, Ti and W; although they are thermodynamically unstable there is a high probability of kinetic stability to reaction with Sb 2 Te 3. Indeed this was demonstrated for Mo which has become a popular practical choice for test cells. Bätzner et al. [26] compares the photovoltaic performance of cells bearing the contact structure Mo/Sb 2 Te 3 /NP-CdTe with others under conditions of accelerated lifetime test (1 sun under V oc at 65 ºC). This contact performs very favourably compared to others, it shows an increase in relative performance up to equivalent (service conditions) lifetimes of approximately 40 years. Further studies of this type of contact are in progress. Although such cells have been made with no J V roll-over the magnitude of the barrier height has not been measured. Quantitative lifetime testing is in its infancy and a particular gap in the literature is the combination of this with quantitative SIMS and carrier concentration measure-

5 phys. stat. sol. (b) 229, No. 2 (2002) 1059 ment. Interplay of the contacts and other parts of the cell are inevitable. Moreover, the behaviour of the front contact is arguably even more poorly understood. 3. Experimental Two batches of 5N CdTe feedstock were purchased from different manufacturers and are referred to here as batch A and batch B. Both were analysed by ICPMS using a Perkin Elmer SCIEX ELAN6000 quadrupole mass spectrometer, samples being dissolved in HF/HNO 3 and aqua regia and diluted prior to analysis. Two groups of nominally identical cells were fabricated from each batch by an all CSS process at ANTEC GmbH. The CdTe was CdCl 2 processed and NP etched (see Section 2.3) prior to contacting with co-sputtered Sb Te and metallisation with V. The final structure was: V(1 mm)/sbte110 nm/npcdte(8 mm)/cds(100 nm)/sno 2 (30 nm)/ito(100 nm)/glass. J V characteristics of the cells were measured under approximate AM1.5 conditions. Further electro-optic characterisation was made at Gent. External quantum efficiency (EQE) was measured as a function of wavelength and applied bias on the device using a chopped system capable of resolving the phase of the photocurrents [27]. AC measurements allowed the phase of the response to be determined. C V profiling was done using the methodology described in [28, 29]. Grain size in the CdTe absorbers was measured by polishing off the contact, etching the grain boundaries with FeCl 3 and recording by SEM. Stability of Sb Te back contacts was investigated as follows: Sb Te was co-sputtered at room temperature onto specially prepared CdTe/CdS/TCO/glass test samples on which the CdTe had been polished flat with bromine/methanol. The CdTe had been CdCl 2 treated but not NP etched. Samples were examined by SIMS in either their asgrown state or after annealing in air at 400 C for 1 h. Quantitative SIMS was done at Mats UK using the CsM + attachment method and was quantified using ion implanted calibration samples on B-face (111) CdTe wafers. 4. Results and Discussion 4.1 Comparative study of cells made with different CdTe ICPMS impurity data for the CdTe from batches A and B are shown in Figs. 2a and b. Data for the dissolution using HF/HNO 3 is shown in preference to that for aqua regia, as dissolution was more complete in the former. Atomic concentrations are shown by atomic number rather Fig. 2. ICPMS impurity concentrations in CdTe listed by atomic number, a) batch A, b) batch B

6 1060 K. Durose et al.: Key Aspects of CdTe/CdS Solar Cells Ta b l e 1 Parameters of cells fabricated from two different batches of 5N CdTe J sc (ma cm 2 ) V oc (mv) FF (%) h (%) CdTe thickness (mm) grain size (mm) impurities >1 ppm batch A Ni, Mo batch B Cu, Zn, Se, Sb, Pb than mass/charge ratio, as the instrument discriminates particular elements by isotope ratio patterns. For any given species slight differences in the reported analyses may be possible if the full range of interferences for example with doubly charged or plasma gas associated species were to be taken into account. Species present in concentrations greater than 1 ppm are identified in Table 1. Batch B feedstock contains higher levels of impurities than does batch A. Average figures taken from the J V data for nine cells from each batch are also shown in Table 1. While the two have comparable short circuit current values, batch B has lower V oc and FF leading to a lower overall efficiency. The grain size and thickness data of the two cell batches has been examined to determine whether the difference might arise from the variation of grain size radius (R) with thickness (h) usually observed in CSS-grown CdTe cells. Using the relation R = h (mm) determined for similar material by Cousins et al. [5], the difference between grain sizes in the two batches can be shown to be significant. Grain size is therefore a potential source of the difference in the performance of the two cell batches. Barna s group [30] has shown that impurities can limit grain growth by impeding step flow mechanisms. The small grain size in the relatively impure batch B is consistent with this but the particular mechanisms responsible have not been identified in this case. Bias dependent quantum efficiency measurements and the phase of the photocurrent response are shown in Figs. 3a and b for a cell from batch B. At zero or low bias conditions the EQE varies little over the range between the band gap energies of CdS and CdTe. This is normal behaviour for the junction region of CdTe/CdS solar cells. The zero phase lag between the light and the photo-response indicates that the collection is in the sense expected for the p n junction. The absence of collection for carriers generated in the CdS has been attributed to high (n + ) doping in the CdS and low (p) doping in the CdTe, i.e. the field is in the CdTe. At the highest value of forward bias (0.7 V) shown in Fig. 3a, the cell is operating in the first quadrant of the J V curve and the expected reversal of the current is revealed by the 180 phase change over the whole wavelength range. However, it is the behaviour at intermediate forward bias values that is of interest here: at 0.4 V the overall EQE declines but a peak at long wavelength grows. Increasing the bias to 0.5 V causes the EQE to rise above 100%. There is a reversed photocurrent for the long wavelength range only. This may be explained as follows. Long wavelength photons penetrate more deeply into the CdTe and generate carriers nearer to the back contact region than short wavelength photons. At high forward bias the p n junction depletion region may be considered to be collapsed, while that for the Schottky contact is in reverse bias and is extended towards the

7 phys. stat. sol. (b) 229, No. 2 (2002) 1061 Fig. 3. Wavelength dependent response of cells made with batch B (less pure) CdTe. Upper part: spectral response (quantum efficiency vs wavelength), b) phase of photocurrent vs. wavelength front surface of the cell by a distance related to the bias and the doping level. The long wavelength EQE results in Fig. 3 are consistent with carrier collection at the back contact under conditions of high forward bias at the junction. Significantly, the cells from batch A show the normal EQE response at low forward bias, but do not show the long wavelength photo-response until somewhat higher forward bias is applied (0.8 V compared to 0.4 V). This might be interpreted as being due to higher doping near the back contact in batch A cells than in batch B cells. However, an alternative explanation would be that photons do not reach the back contact depletion region in cells from batch A since the CdTe is slightly thicker than in the other batch. (For a full explanation of EQE phenomena and their role in interpreting the differences between light and dark J V curves, the reader is referred to [27].) Further evidence concerning the carrier concentration in the cells is presented in the form of effective carrier concentrations determined from C V profiling as shown in Fig. 4. The method described in [29] yields effective carrier concentrations for both the near junction and back contact regions of the absorber. Figure 4 clearly shows the effective carrier concentration of batch A cells to be greater than that of batch B cells. In particular doping near the back contact of batch B cells is reduced from to cm 3. Hence both the EQE and C V measurements give an indication that the carrier concentrations near the back contacts in the two batches of cells differ, with batch A being more highly doped than batch B. This being the case, then the contact depletion region of batch B should be wider than for batch A cells, leading to the lower performance of batch B devices. From the ICPMS data we might infer that the less pure sample (batch B) is more heavily compensated than batch A and it is this that leads to a decrease in performance, rather than for example a difference in back contact barrier height.

8 1062 K. Durose et al.: Key Aspects of CdTe/CdS Solar Cells Fig. 4. Carrier concentrations determined from the local slope of the 1/C 2 V curve for cells made from batch A and batch B CdTe. Distance is measured from the back contact position at 0, the p n junction being on the right-hand side of the plots Back contact diffusion study SIMS profiles for Sb and O in Sb Te/CdTe/CdS/ TCO/glass layers are shown in Figs. 5a and b for the as-deposited and annealed samples, respectively. In the as-deposited samples the background Sb level in the CdTe is cm 3. Oxygen concentrated at a spike at the Sb Te/CdTe interface is probably present due to surface oxidation of the CdTe prior to coating. In the bulk of the layer there is a high level of O (10 19 cm 3 ). Air annealing (Fig. 5b) promotes a strong influx of Sb associated with O to a depth of about 1 mm. Reasons for this concurrent influx may be determined from the free energies of reactions in the Sb Cd Te O system as shown on the tetrahedral phase diagram due to Schmidt et al. [24]. This indicates that at room temperature trace Sb with O in CdTe is stable as Sb 2 O 3. New work presented here extends the calculations by including Sb 2 O 5 and assessing the temperature dependence of reactions in the system by evaluating DG(T) using DG = DH T DS. The thermo-chemical data used are shown in Table 2 (no literature value of C p is available for CdTeO 3 and so the values were estimated). The reactions considered are shown with their calculated free energy variations in Fig. 6. It can be seen that oxidation reactions of Sb and Sb 2 Te 3 are always favoured over those of CdTe and Te. Hence it may be proposed that during air annealing Sb diffuses into the CdTe along grain boundaries, and the driving force for O ingress is reaction with Sb to form its oxides, the most thermodynamically stable being Sb 2 O 5. atomic concentration (cm 3 ) (a) as deposited Oxygen Antimony depth from surface (µm) atomic concentration (cm 3 ) (b) air annealed Oxygen Antimony depth from surface (µm) Fig. 5. Quantitative SIMS profiles of the contact region of Sb Te/CdTe/CdS/TCO/glass test structures. a) as-deposited, b) air annealed at 400 C

9 phys. stat. sol. (b) 229, No. 2 (2002) 1063 Ta b l e 2 Thermo-chemical data used in the calculation of DG f (T) in Fig. 6 material DH 0 f (kj/mol) S 0 (J/K 1 mol 1 ) DG 0 f (kjmol 1 ) C p (JK 1 mol 1 ) O Cd CdO Te TeO CdTe CdTeO ? Sb Sb 2 O 3 cubic Sb 2 O 3 orthorhobic Sb 2 O Sb 2 O Sb 2 Te Real cells are unlikely to meet such extreme conditions. The relevance of these reactions to encapsulated cells probably lies in the reaction of Sb with O in the CdTe during ageing. 5. Conclusions ICPMS analysis of 5N CdTe from two different suppliers shows them to have rather different impurity content. Nominally identical cells fabricated from the two feedstocks yielded cells with different performance parameters. Cells made from the less pure batch (B) had degraded V oc, FF and h and a lower grain size. Quantum efficiency and C V data indicate that cells made with the less pure feedstock have lower doping at the back contact than the pure material. This might be the result of impurity compensation and would act to reduce performance. An alternative explanation the influence of grains cannot be discounted, but grain size itself is known to be influenced by impurities. Relatively extreme air annealing has been shown by SIMS to cause Sb and O to diffuse into CdTe from Sb Te contacts. Free energy calculations show that the O is DG (10 2 kj mol -1 ) T (K) Fig. 6. Temperature dependence of the free energies of presumed reactions in the O Sb Cd Te system. (1) 2Sb + 3Te! Sb 2 Te 3 ; (2) Te + O 2! TeO 2 ; (3) CdTe + 1.5O 2! CdO + TeO 2 ; (4) CdTe + 1.5O 2! CdTeO 3 ; (5) 2Sb + 1.5O 2! Sb 2 O 3 orthorhombic ; (6) 2Sb + 1.5O 2! Sb 2 O 3 cubic ; (7) 2Sb + 2O 2! Sb 2 O 4 ; (8) 2Sb + 2.5O 2! Sb 2 O 5 ; (9) Sb 2 Te O 2! Sb 2 O 3 orthorhombic + 3TeO 2 ; (10) Sb 2 Te O 2! Sb 2 O 3 cubic + 3TeO 2 ; (11) Sb 2 Te 3 + 5O 2! Sb 2 O 4 + 3TeO 2 ; (12) Sb 2 Te O 2! Sb 2 O 5 + 3TeO 2

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