TSF-31481; No of Pages 5 Thin Solid Films xxx (2013) xxx–xxx
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Comparative study of Cu(In,Ga)Se2/CdS and Cu(In,Ga)Se2/In2S3 systems by surface photovoltage techniques Th. Dittrich a,⁎, A. Gonzáles a, b, T. Rada a, b, T. Rissom a, E. Zillner a, S. Sadewasser a, c, M. Lux-Steiner a a b c
Helmholtz-Zentrum Berlin für Materialien und Energie, Hahn-Meitner-Platz 1, D-14109 Berlin, Germany Departamento de Física, Universidad del Norte, km 5 Via Pto Colombia, Barranquilla, Colombia International Iberian Nanotechnology Laboratory, Avda. Mestre José Veiga s/n, 4715-330 Braga, Portugal
a r t i c l e Available online xxxx Keywords: Surface photovoltage Chalcopyrite solar cell Hetero-junction
i n f o
a b s t r a c t Cu(In,Ga)Se2 absorbers were investigated by surface photovoltage (SPV) in the Kelvin probe and fixed capacitor arrangements before and after deposition of CdS or In2S3 buffer layers as well as before and after deposition of ZnO window layers. Effects such as passivation of surface states, partial electron transfer from ZnO into In2S3, decrease of the ideality factor after deposition of ZnO and slow electron transfer through In2S3 were demonstrated. The results show that SPV measurements open opportunities for dedicated studies of charge separation at hetero-junctions between ordered and disordered semiconductors. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Hetero-junctions between ordered and disordered semiconductors are important for the formation of charge-selective contacts in solar cells based, for example, on chalcopyrite [1], CdTe [2], kesterite [3] or crystalline silicon [4] absorbers. Especially the interface between the so-called buffer layer and chalcopyrite absorbers plays a crucial role for the performance of solar cells [5]. CdS buffer layers are widely used while alternative buffer layers such as In2S3 are under development. We remark that the electronic properties of a hetero-junction between ordered and disordered semiconductors crucially depend on electronic defect states at the interface and in the disordered layer. In this sense, the disordered layer can be understood as an amorphous or nanocrystalline semiconductor layer. A deeper understanding of electronic states and transport processes in hetero-junctions between ordered and disordered semiconductors is needed for further improvements in the performance of related solar cells. This work is aimed to show the opportunities of surface photovoltage (SPV) techniques on the example of comparative studies of charge separation in ZnO/CdS or In2S3 buffer/Cu(In,Ga)Se2 absorber systems while the bare absorber, the absorber coated with the buffer layer and the complete system were investigated. Especially the deposition of In2S3 buffer layers with the non-conventional ILGAR (ion layer gas reaction [6]) technique is of great interest due to a relatively large experimental degree of freedom under excellent control of deposition parameters besides process scalability. For example, it has been shown by Fu et al. that modifications of the ILGAR deposition process of the In2S3 buffer layer open the opportunity for further increase of VOC of chalcopyrite solar cells [7,8]. On the other ⁎ Corresponding author. E-mail address:
[email protected] (T. Dittrich).
hand, the role of residual chlorine in In2S3 layers deposited by ILGAR from InCl3 precursor salt solutions [9] for the formation of contacts is not well understood yet. SPV [10] signals arise whenever photo-generated charge carriers are separated in space. SPV signals are highly sensitive to those spatial regions where charge separation takes place. The sign of SPV signals gives information about the preferential direction of charge separation, it is, for example, negative for depleted p-type semiconductors. Further, SPV signals are a measure for band bending in conventional crystalline semiconductors and spectral dependencies can give information about defect states below the band gap. SPV signals can be measured from dc (Kelvin-probe arrangement) to times as short as ns (fixed capacitor arrangement, [11], see also Fig. 1). The influence of different mechanisms of charge separation, transport and relaxation on SPV signals make their interpretation often rather complicated. In the past, SPV measurements in the Kelvin probe arrangement were applied to quality control of Cu(In,Ga)Se2-based thin film solar cells [12]. In this work, for example, a strong correlation of the sodium content with SPV signals was shown. In the Kelvin-probe arrangement the SPV signal is equivalent to the open circuit voltage (VOC) of a corresponding solar cell. Therefore the behavior of parameters such as ideality factor (n), diode saturation current density (I0) and shunt resistance (Rp) can be investigated by intensity dependent SPV measurements at various preparation steps (Eq. (1), ISC — short circuit current density). ISC ¼ I0
q VOC VOC þ exp n kB T Rp
ð1Þ
Examples will be given for SPV measurements under different measurement conditions and ways for interpretation of effects observed on ZnO/CdS/(In,Ga)Se2 or In2S3/Cu(In,Ga)Se2 systems will be drawn.
0040-6090/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2012.12.078
Please cite this article as: T. Dittrich, et al., Thin Solid Films (2013), http://dx.doi.org/10.1016/j.tsf.2012.12.078
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Fig. 1. Schematic energy diagrams of an electrode with a photoactive surface layer shunted with a reference electrode in the dark (a) and under illumination (b), of the same electrodes under illumination but with an external bias potential (c) or with a large measurement resistance between both electrodes (e). The photographs show the Kelvin-probe (d) and of the fixed capacitor arrangement (f) used in the experiments. The mica sheet between the sample and reference electrodes is omitted in (e). Evac, EFs, EFref, Ws, Wref, CPDdark, ΔCPD, q, Vb, and Uph denote the vacuum energy, the Fermi-level of the electrode of the sample, the Fermi-level of the reference electrode, the workfunction of the electrode with the sample, the workfunction of the reference electrode, the contact potential difference in the dark, the light induced change of the contact potential difference, the elementary charge, the value of the external potential, and the photovoltage, respectively. In the Kelvin-probe arrangement the reference electrode is vibrating while the ac current is adjusted to zero with the external potential. The respective external potential is the sum of CPDdark and ΔCPD. In the fixed capacitor arrangement the Fermi-levels of the sample and reference electrodes are aligned via a large resistance before illumination and the surface photovoltage signal is coupled out with a high impedance buffer. Measured surface photovoltage signals correspond to the negative light induced change of the contact potential difference.
2. Experimental details Cu(In,Ga)Se2 (CIGSe) layers were deposited by a standard process [13] on glass substrates coated with molybdenum. Five samples were investigated after different preparation steps. First, all samples were etched in KCN. Sample 1 was not further processed after KCN etching. In the following CdS was deposited by chemical bath deposition (samples 2 and 4) and In2S3 by ILGAR with an InCl3 precursor salt solution (samples 3 and 5) [6], respectively. Samples 2 and 3 were not further processed after deposition of the buffer layer. Finally, undoped and doped ZnO layers were sputtered on top of CdS (sample 4) or In2S3
(sample 5) buffer layers. The samples were kept in inert gas atmosphere after preparation. SPV measurements were performed in the Kelvin-probe and fixed capacitor arrangements [10] (see also Fig. 1). In the Kelvin-probe arrangement the ac current arising between the vibrating reference (Besocke deltaPHI) and the grounded sample electrodes is adjusted to zero with an external dc potential which is equal to the contact potential difference (CPD) for zero current. The light induced change of CPD (ΔCPD) corresponds to the negative (dc) SPV. In the fixed capacitor arrangement the SPV is measured as the time dependent voltage drop across a large measurement resistance (Rm = 10 GΩ)
Please cite this article as: T. Dittrich, et al., Thin Solid Films (2013), http://dx.doi.org/10.1016/j.tsf.2012.12.078
3. Results and discussion 3.1. Spectral dependent dc SPV Fig. 2 shows CPD spectra of samples 1–5. The values of CPDdark were quite similar for all samples except sample 3 (bare In2S3 surface) for which the CPDdark was more negative by about 0.5 V (see also Fig. 3 (a), CPDdark was obtained at the beginning of the spectral dependent measurements). The sign of the ΔCPD was positive for all samples, i.e. electrons were separated towards the external surface as expected for a depleted p-type semiconductor. The value of ΔCPD showed the strongest increase at photon energies between 1.0 and 1.1 eV, i.e. near the band gap of CIGSe, while the maximum of ΔCPD was measured at about 1.2 eV. The negative values of the maximum dc SPV are given in Fig. 3 (b) for the different surface treatments and surface layer depositions. The SPV was −48 mV for sample 1 and increased to −194 and −334 mV after deposition of CdS (sample 2) or In2S3 (sample 3), respectively, i.e. the band bending increased stronger after deposition of the In2S3 than after deposition of the CdS buffer. After deposition of ZnO the SPV increased to −422 mV for the CdS buffer (sample 4) but decreased to −292 mV for the In2S3 buffer (sample 5). Mo / CIGSe
/ KCN etched / CdS / In2S3
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between the reference and sample electrodes, while the capacitance (C) between both electrodes is defined by a mica sheet [14]. Modulated or pulsed excitation (EKSPLA NT-342/1/UVE, pulse duration time: 5 ns, excitation wavelength: 660 nm, pulse intensity: 15 μJ/cm 2, sampling rate: 100 MHz [11]) were applied for SPV measurements with the fixed capacitor. As remark, the RmC time constant of the system was about 0.1 s. The in-phase (x) and phase-shifted by 90° (y) signals (with respect to the modulated light) were measured with a double phase lock-in amplifier (EG&G 5210). The amplitude and the tan of the phase angle are defined as the square root of the sum of the squared x and y signals and as the ratio between the y and x signals, respectively. Frequencies (fmod) of 6 and 20 Hz were chosen for modulation. The phase was calibrated with a Si photodiode. A halogen lamp with a quartz prism monochromator served as a light source for spectral dependent measurements. Intensity dependent measurements with the Kelvin-probe were performed with intense light emitting diodes (LEDs, red or RGB-LED light source with ultra-bright red, green and blue LEDs together) in on–off cycles. The electrodes were installed in a homemade vacuum chamber (pressure 10−3 Pa). The samples were investigated immediately after preparation. The active surface area of the samples with ZnO was adjusted to the diameters of the electrodes (3 or 6 mm for Kelvin probe of fixed capacitor, respectively) to avoid disturbance due to lateral charge transport.
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Fig. 3. Dependencies of the contact potential difference in the dark (a) and of the maximum light induced change of the contact potential difference (b) regarding Fig. 2 for the various surface treatments and surface layer depositions.
Therefore, the initially higher band bending at the In2S3/CIGSe interface was partially compensated by an electron transfer from ZnO into In2S3 what seems plausible with respect to the more negative CPD in the dark for sample 3. As a consequence the work function of the In2S3 ILGAR layer has to be modified. The ΔCPD was relatively high for samples 2 and 3 at photon energies below the onset of the fundamental absorption but decreased strongly in this region after deposition of ZnO. This gives evidence for passivation of defect states at the CdS and In2S3 surfaces by ZnO. A pronounced decrease of ΔCPD at photon energies above the band gap of CdS was observed for samples 2 and 4 showing that photo-generation in the CdS layer did not contribute or contributed significantly less than generation in the absorber layer to dc charge separation. 3.2. Intensity dependent dc SPV The intensity dependencies of the SPV signals of samples 3, 4 and 5 are shown in Fig. 4 (inset gives an example for on-off cycling with increasing intensity). The dependencies were fitted with Eq. (1) while a linear intensity dependence of ISC was assumed. The ideality factors were 1.8 and 1.3 for samples 3 and 5, respectively. This means that passivation of surface defect states by ZnO as mentioned above caused a strong decrease of the ideality factor, i.e. led to a change of the dominating recombination mechanism. The value of Rp decreased more than 3 orders of magnitude after deposition of ZnO whereas the diode saturation current density remained practically unchanged after deposition of ZnO in In2S3. The value of I0 was about 20 times less for sample 4 than for sample 5. The absolute value of I0 can be estimated if assuming a reasonable value of ISC at maximum intensity of 30 mA/cm 2 which is of the order of ISC for CIGSe solar cells illuminated at AM1.5. The value of I0 would be about 2 · 10 −8 A/cm 2 for sample 4 in this case.
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The spectra of the modulated PV amplitude are depicted in Fig. 5 (a) for samples 1, 2, 4 and 5. The positions of the maximum (arrow B) were very similar for these spectra. The modulated SPV decreased in the defect region below the band gap after deposition of CdS giving evidence for surface defect passivation at the CIGSe surface. Further, the modulated SPV decreased strongly (to the noise level) after deposition of ZnO pointing to a very efficient surface defect passivation at the CdS surface by ZnO. Interestingly, the amplitude of the modulated SPV
Please cite this article as: T. Dittrich, et al., Thin Solid Films (2013), http://dx.doi.org/10.1016/j.tsf.2012.12.078
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increased for photon energies above the band gap of CdS for sample 2 but not for sample 4. Therefore the CdS participated in the modulated charge transfer due to defect states at the surface of the bare CdS. It is interesting to point on the decrease of the PV amplitude of sample 5 in comparison to sample 4 at photon energies especially above 2 eV and on the dip in the spectrum of sample 5 at photon energies between 2.5 and 2.6 eV. This behavior gives evidence for an additional mechanism of charge separation in sample 5 which has to be related to absorption in the In2S3 layer. The additional mechanism of charge separation has the tendency to decrease the PV amplitude arising from charge separation in the space charge region of the chalcopyrite absorber.
The spectra of the phase angle are given in Fig. 5 (b) for samples 1, 2, 4 and 5. A phase angle just below 180° means that electrons are separated towards the external surface with a retardation in relation to the modulation period. The phase angles were practically constant at photon energies between 1.1 and 2.1 eV (about 175° and 145° for fmod = 6 or 20 Hz, respectively). A phase angle close to 180° means that charge separation and following relaxation are fast in comparison to the modulation period. The phase angles of the defect states (A) were 150° and 165° for samples 1 and 2. This points to an increase of slower relaxation processes for modulated charge separation from defect states. A clear change of the phase angle for light absorption in CdS (C) was observed for sample 2 but disappeared for sample 4. For sample 5 the phase angles of defect states changed to values of about − 30°, i.e. electrons were separated preferentially towards the internal interface. This was also the case for excitation at higher photon energies (C) when photons were mainly absorbed in the In2S3 buffer layer. The change of the sign of the modulated SPV signal can be directly seen in the in-phase and phase-shifted by 90° spectra shown in Fig. 6 for samples 4 and 5. The spectra of samples 4 and 5 had the same sign and looked quite similar in the full range of SPV signals (a) since the signals were not well resolved at the linear scale for photon energies below the band gap of the chalcopyrite and above the band gap of In2S3. The opposite signs of the spectra of samples 4 and 5 became obvious for photon energies below the band gap of the chalcopyrite and above 2.5 eV after zooming into the signal range of SPV signals by a factor of 200 (b). There was no change of the sign for sample 3 (not shown). Therefore, the additional mechanism of charge separation was caused by the formation of a little barrier at the ZnO/In2S3 contact. Modulated charge separation at this barrier became dominating for excitation from defect states or for strong absorption in the In2S3 layer. The formation of the little barrier at the ZnO/In2S3 interface (i) leads to a stronger decrease of the modulated PV amplitude with increasing photon energy for sample 5 in comparison to sample 4 and (ii) was also the reason for the decrease of the SPV signal measured in the Kelvin probe arrangement.
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3.4. Transient SPV SPV transients of samples 4 and 5 are given in Fig. 7 for photogeneration in CIGSe. The energy of exciting photons (1.88 eV) was below the band gap of CdS (2.4 eV) and In2S3 (2.2 eV [10]) so that pulsed illumination lead to the photo-generation of electron–hole pairs in the chalcopyrite absorber. The SPV signals arose immediately with the light pulse giving evidence for fast charge separation in the space charge region of CIGSe. Additional retarded components were observed for both transients while the retardation time was much shorter for sample 4 (50 ns) than for sample 5 (400 ns). The retardation was caused by slow electron transport [15] through the CdS or In2S3 layers towards the interface with ZnO while the transport was much slower in In2S3. The quite strong retardation in relation to the thickness of the buffer layer is a signature for a low effective electron diffusion coefficient in the In2S3 layer (of the order of 10−3 … 10−4 cm2/s). For more detailed interpretation the thickness of the buffer should be varied and intensity and temperature dependent measurements should be performed. At longer times the transients were very similar and decayed within 150 or 260 μs for samples 4 or 5, respectively. 4. Conclusions Opportunities of SPV analysis for the investigation of electronic properties of buffer layers and interfaces in chalcopyrite solar cells were demonstrated on the examples of dc spectral dependent, intensity dependent dc, modulated spectral dependent and transient SPV. Each of the applied SPV techniques gave specific information about charge separation at chalcopyrite surfaces depending especially on the nature of the buffer layer. The most striking difference between the CdS and In2S3 buffer layers was that the higher dc SPV signal was reached for the chalcopyrite absorber with In2S3 before subsequent sputtering of ZnO and that the dc SPV signal increased for the CdS buffer layer but decreased for the In2S3 buffer layer after deposition of the ZnO window layer. In contrast to the CdS buffer, the decrease of the dc SPV signal of the chalcopyrite absorber with the In2S3 buffer layer after deposition of ZnO was accompanied by a change of the sign of the modulated SPV signals in the spectral ranges of absorption by defect states below the band gap or of dominant absorption by In2S3 at photon energies above 2.6 eV. This gives evidence for the formation of a little barrier
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Fig. 7. Examples for surface photovoltage transients of Mo/CIGSe/CdS/ZnO and Mo/ CIGSe/In2S3/ZnO.
for electrons at the In2S3/ZnO junction, i.e. for a non-ideal Ohmic behavior. We speculate that this little barrier is related to accumulation of residual chlorine [13] at the ZnO/In2S3 interface. SPV techniques are powerful in general for the investigation and monitoring of interfaces in systems with hetero-junctions between ordered and disordered semiconductors. This contact-less and nondestructive measurement technique should be considered as a candidate for quality control during the different steps of solar cell manufacturing. Acknowledgment The authors are grateful to N. Allsop for ILGAR preparation, to C.-H. Fischer and Y. Fu for discussions around ILGAR and to the two reviewers for constructive and detailed criticism. References [1] P. Jackson, D. Hariskos, E. Lotter, S. Paetel, R. Wuerz, R. Menner, W. Wischmann, M. Powalla, Prog. Photovolt. Res. Appl. 19 (2011) 894. [2] X. Wu, Sol. Energy 77 (2004) 803. [3] D.A.R. Barkhouse, O. Gunawan, T. Gokmen, T.K. Todorov, D.B. Mitzi, Prog. Photovolt. Res. Appl. 20 (2012) 6. [4] Y. Tsunomura, Y. Yoshimine, M. Taguchi, T. Baba, T. Kinoshita, H. Kanno, H. Sakata, E. Maruyama, M. Tanaka, Sol. Energy Mater. Sol. Cells 93 (2009) 670. [5] R. Herberholz, V. Nadenau, U. Rühle, C. Köble, H.W. Schock, B. Dimmler, Sol. Energy Mater. Sol. Cells 49 (1997) 227. [6] N.A. Allsop, A. Schönmann, A. Belaidi, H.-J. Muffler, B. Mertesacker, W. Bohne, E. Strub, J. Röhrich, M.C. Lux-Steiner, C.-H. Fischer, Thin Solid Films 513 (2006) 52. [7] Y.P. Fu, N.A. Allsop, S.E. Gledhill, T. Köhler, M. Krüger, R. Saez-Araoz, U. Bloeck, M.C. Lux-Steiner, C.-H. Fischer, Adv. Energy Mater. 1 (2011) 561. [8] Y.P. Fu, T. Rada, C.-H. Fischer, M. Lux-Steiner, Th. Dittrich, Prog. Photovolt. Res. Appl. in press, http://dx.doi.org/10.1002/pip.2305. [9] an example for the chlorine content in In2S3 layers deposited by ILGAR with InCl3 precursor is shown in. C.-H. Fischer, N.A. Allsop, S.E. Gledhill, T. Köhler, M. Krüger, R. Saez-Araoz, Y. Fu, R. Schwieger, J. Richter, P. Wohlfahrt, P. Bartsch, N. Lichtenberg, M.C. Lux-Steiner, Sol. Energy Mater. Sol. Cells 95 (2011) 1518. [10] see the review of. L. Kronik, Y. Shapira, Surf. Sci. Rep. 37 (1999) 1. [11] see, for example. Th. Dittrich, S. Bönisch, P. Zabel, S. Dube, Rev. Sci. Instrum. 79 (2008) 113903. [12] L. Kronik, B. Mishori, E. Fefer, Y. Shapira, W. Riedl, Sol. Energy Mater. Sol. Cells 95 (1998) 21. [13] see, for example. D. Abou-Ras, J. Dietrich, J. Kavalakkatt, M. Nichterwitz, S.S. Schmidt, C.T. Koch, R. Caballero, J. Klaer, T. Rissom, Sol. Energy Mater. Sol. Cells 95 (2011) 1452, (and refs. therein). [14] V. Duzhko, V.Yu. Timoshenko, F. Koch, Th. Dittrich, Phys. Rev. B 64 (2001) 075204. [15] Th. Dittrich, I. Mora-Seró, G. Garcia-Belmonte, J. Bisquert, Phys. Rev. B 73 (2006) 045407.
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