- Email: [email protected]
Electrical aspects of operation of polymer–fullerene solar cells
Thin Solid Films 451 – 452 (2004) 493–497
Electrical aspects of operation of polymer–fullerene solar cells Vladimir Dyakonov* Energy and Semiconductor Research Laboratory, Institute of Physics, University of Oldenburg, D-26111 Oldenburg, Germany
Abstract Easily processable conjugated polymers for optoelectronics, in general, and for photovoltaic energy conversion, in particular, is an attractive research field, in which the combined efforts of material science, device engineers, and spectroscopists are welcomed. To improve the efficiency of polymer solar cells, it is vital to understand which mechanisms control the current–voltage characteristics of a given device. The current–voltage characteristics of ITOyPEDOT:PSSyOC1 C10 -PPV:PCBMyAl solar cells were measured in the temperature range 120–320 K under variable illumination, between 0.03 and 100 mWycm2 (white light). The short circuit current density grows monotonically with temperature until 320 K. This is indicative of a thermally activated transport of photogenerated charge carriers, influenced by recombination with shallow traps. A gradual increase of the open circuit voltage to 0.91 V was observed upon cooling the devices down to 120 K. The overall effect of temperature on solar cell parameters results in a positive temperature coefficient of the power conversion efficiency, which is 1.9% at Ts320 K and 100 mWycm2. The nearly linear variation of the short circuit current density with light intensity confirms that the internal recombination losses are predominantly of a monomolecular type under short circuit conditions. 䊚 2003 Elsevier B.V. All rights reserved. PACS: 72.20.Jv; 72.40.qw; 72.80.Le; 73.40.Sx; 73.61.Ph; 85.30.De Keywords: Polymer semiconductors; Photovoltaic effect; Charge carriers trapping
1. Introduction The use of conjugated polymers and molecules exhibits a high potential for production of efficient, and at the same time, low-cost, flexible optoelectronic devices with the option for large area applications w1x. The expanding field of organic electronic devices has stimulated a lot of attention and research on fundamental questions concerning the transport properties of electrically conducting polymers as well as the metalyorganic semiconductor interfaces. For photovoltaic applications, high photogeneration yield (generation of electrons and holes due to light absorption) is important w2x. One approach to achieve an efficient charge carrier generation in polymer light absorbers is to blend them with suited acceptors. For example, in polymer–fullerene composites, the conjugated polymer acts as a donor, and the fullerene molecule as an acceptor. One of the most effective and therefore extensively studied device concepts, so far, is based on the bulk heterojunction approach w3,4x. In this, the fullerene molecules are *Tel.: q49-441-7983538; fax: q49-441-7983326. E-mail address: [email protected] (V. Dyakonov).
dispersed in a polymer or in a low molecular weight organic matrix. The thin photoactive film is then sandwiched between two electrodes with asymmetric work functions. In this configuration, each electrode is expected to form an ohmic contact with the respective p-, or n- type semiconductor, while simultaneously blocking the minority charge carriers. For single junction solar cells based on the polymer bulk heterojunction concept, a power conversion efficiency of 2.5% has been achieved under 1 sun AM1.5 illumination conditions without light trapping w5x. This value is still quite modest for applications. However, much higher development potential can be anticipated taking the high quantum yield of charge carrier generation in these composites into account. In spite of significant advances in the understanding of the qualitative behaviour of organic solar cells, a fully quantitative description of charge injection at the electrodes, charge transport in the bulk, and electron-hole recombination has yet to emerge. This indicates the importance of the electrical transport properties of the absorber material and the interfaces. In order to further develop the bulk heterojunction concept, as well as to identify factors limiting the solar cell’s
0040-6090/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2003.11.063
494
V. Dyakonov / Thin Solid Films 451 – 452 (2004) 493–497
performance, a detailed understanding of the underlying device physics is imperative. We studied the influence of temperature and illumination intensity on the principal parameters of the polymer-fullerene bulk heterojunction solar cells, such as short circuit current and open circuit voltage. 2. Experimental The devices studied in this work were made in a ITOyPEDOT:PSSyOC1 C10 -PPV:PCBMyAl configuration. The active layer was made by spin coating a 1:4 chrolobenzene solution of poly w2-methoxy-5- (3-,7dimethyl-octyloxy)-1,4-phenylene vinylenex (OC1C10PPV) and phenyl-C61 butyric acid methyl ester (PCBM) onto a conducting substrate. The OC1C10-PPV was synthesized via the sulfinyl route as described elsewhere w6x. As a hole extracting electrode, the 80-nm layer of poly-wethylene dioxy thiophenex doped with polystyrene sulfonate (PEDOT: PSS), BAYTRON P, (Bayer AG, Germany) was spin coated on a glass substrate, coated with patterned indium tin oxide. The aluminium counter electrode was ther˚ mally evaporated in a vacuum chamber at rate 4 Ays. The current–voltage measurements were carried out in a variable temperature cryostat equipped with a liquid nitrogen supply and a temperature controller (Lakeshore 330) with a source-meter unit (Avantest TR-6143). The devices were illuminated through a sapphire window by white light from a xenon arc lamp whose intensity is controlled by a set of neutral density filters with transmission coefficients from 10y4 to 1. The maximum intensity was calibrated to 100 mWycm2 inside the cryostat. 3. Results and discussion Current density–voltage (J–V) measurements were performed in the temperature range Ts120–320 K in the dark, and under illumination for incident light intensities PLight from 0.03 to 100 mWycm2. The solar cell parameters, i.e. the open circuit voltage VOC, the short circuit current density JSC, the fill factor FF, and the power conversion efficiency h were determined. 3.1. Temperature dependence 3.1.1. Short circuit current Fig. 1 shows the short circuit current density JSC vs. temperature under different illumination intensities. The short circuit current density of the studied samples increases with temperature. The strong temperature effect we observed may be due to the electronic transport properties of the absorber material, e.g. due to variation of mobility. Further, it is negatively influenced by the capture of charge carriers by traps w7x. In both cases,
Fig. 1. Short circuit current density JSC as a function of temperature at light intensities (white light) indicated in the legend. Dashed lines represent calculated values with Eq. (1). For all light intensities we obtained the activation energy of Ds45 meV. Active layer thickness ds80 nm. Polymer–Fullerene weight ratio is 1:4.
one expects that an increase in temperature will promote the current through the device. In the absence of monoor bimolecular recombination, the JSC is expected to be weakly temperature dependent, mainly due to thermal generation of charge carriers across the semiconductor band gap. In this case, the thermally activated variation of mobility should not influence the steady state photogenerated current. The total amount of photogenerated charges will remain unaffected. Instead, the capture and emission of charge carriers by shallow traps may lead to a variation of the current with temperature. This is a thermally activated process and is denoted as monomolecular recombination. To prove this, the experimental data in Fig. 1 were fitted with the expression: B
JSCŽT,PLight.sJ0ŽPLight.expCy D
DE F, kT G
(1)
where J0(PLight) is a pre-exponential factor, which is comprised of the thermally generated charge carrier density, their mobility, and the electric field. Finally, D is the trap depth with respect to the corresponding band. From the fits we obtained the activation energy Ds45 meV for all the light intensities applied, and a strictly linear dependence for the pre-exponential factor with light intensity (not shown). We note that this interpretation is only valid in case of monomolecular recombination of photogenerated charges. In case of bimolecular recombination, e.g. due to the space charge effect, the thermally activated mobility will be responsible for the temperature dependence of the short circuit current, provided the traps are filled. The occurrence of bimolecular recombination can be established in a square root current density–intensity relationship.
V. Dyakonov / Thin Solid Films 451 – 452 (2004) 493–497
495
is the acceptor density, m is the mobility. Therefore, VOCs
Fig. 2. Open circuit voltage VOC as a function of temperature under different illumination intensities (white light) as indicated in the legend.
3.1.2. Open circuit voltage The origin of the open circuit voltage in bulkheterojunction solar cells is a matter of discussions. Physically the VOC is controlled by the splitting of quasiFermi levels in the absorber. It was found to correlate directly with the acceptor strength, i.e. it is determined by the HOMO-LUMO energy gap of donor and acceptor, respectively, being nearly independent of electrode materials w8x. In a metal-insulator-metal (MIM) picture w9x, the VOC is limited by the work function difference between electrodes, i.e. is equal to the built-in field. In Ref. w10x, the VOC is expected to be determined by both the built-in field and the photoinduced contribution, i.e. by the electrochemical potential gradient across the absorber layer. The open circuit voltage was measured as function of temperature at different illumination intensities and is shown in Fig. 2. At Ts300 K and the highest illumination intensity used, PLights100 mWycm2, the VOC is 0.82 V. Upon cooling down the sample to Ts120 K, the VOC increases to 0.91 V. For conventional inorganic solar cells an increase in VOC over a whole temperature range is expected w11x. The temperature dependence of VOC is based on Shockley equation for an illuminated p–n solar cell: w
B
y
D
JsJ0xexpCy
qV E z Fy1|yJL, kT G ~
E kT B JSC Eq kT B NVNCmkT E F. lnC q1Ff y lnC G q D J0 q q D JSCLNA G
(4)
In limit T™0 K, the VOC approaches Eg yq, where Eg is the band gap energy of the absorber material, q is the elementary charge. The applicability of such an approach to polymer solar cells is unclear, as the temperature dependence of several important parameters, i.e. of diffusion length and acceptor density, is unknown. However, one can see the tendency of curves to converge at low temperatures. Already the room temperature value is higher than the one expected from MIM picture, i.e. 0.7 V for Al and PEDOT:PSS, and seems to be determined andyor limited by the energetic gap between the polymer’s HOMO and the LUMO of the acceptor material, 1.2 eV. This feature of polymer solar cells was observed for many different composites used in the absorber, in some cases showing VOC well above the difference in the work functions of the electrodes, already at room temperature w12,13x. 3.2. Illumination intensity dependence The charge carriers in the solar cells under investigation are efficiently photogenerated via ultrafast electron transfer between donor and acceptor counterparts of the composite, usually having a very low charge carrier concentration in the dark. The fate of the photogenerated electrons and holes is crucial for the device efficiency, therefore, the influence of light intensity on the J–V characteristics is important. 3.2.1. Short circuit current and open circuit voltage Fig. 3 displays the short circuit current density as function of light intensity PLight in a double-logarithmic
(2)
where JL is photogenerated current density, JsJLfJSC at Vs0, and J0 is saturation current density: B yE E
J0sNVNCmkTexpC D
1 , kT G LNA g
F
(3)
where NVyC is the effective density of states in the valenceyconduction band, L is the diffusion length, NA
Fig. 3. Short circuit current density as a function of light intensity for different temperatures. The scaling exponents (JSCAPaLight) between 0.92 and 0.85 are obtained for Ts320 K and Ts120 K, respectively.
496
V. Dyakonov / Thin Solid Films 451 – 452 (2004) 493–497
Fig. 4. Open circuit voltage as a function of light intensity in a semilog scale at temperatures between 120 and 320 K.
scale. The JSC follows the power law dependence JSCs PaLight with scaling exponents as0.92 and 0.85 for Ts 320 K and Ts120 K, respectively. The nearly linear dependence of the JSC indicates that charge carrier losses in the absorber bulk are dominated by the monomolecular recombination in defects or impurities. The bimolecular e-h recombination was not observed even at highest illumination intensity. This is in agreement with the picture of photogeneration and transport of charge carriers in a donor–acceptor bulk heterojunction: an ultrafast e-h separation is followed by the transport of negatively and positively charged carriers within spatially separated networks: electrons along percolated fullerenes travel towards the Al contact, holes within the polymer matrix travel towards the PEDOT:PSS contact. As both contacts may be considered ohmic for the respective charge carriers (i.e. the potential barriers are below 0.5 eV), no space charge limited conditions under short circuit conditions, even at highest generation rates, exist. The absence of bimolecular recombination together with a strong temperature dependence of the short circuit current confirm our assumption that the efficiency of devices is limited by the charge transport properties of the absorber materials, i.e. by recombination with shallow traps. We note that this is valid by assuming a temperature independent charge carrier generation rate. Provided the short circuit current density is proportional to the light intensity (see Fig. 3) and the saturation current density J0 is independent of the illumination level, the VOC is expected to be proportional to ln(JSC). This behaviour (Fig. 4) was found in the higher temperature range, Ts280–320 K, only, whereas at lower temperatures all curves saturate at intensities between 1 and 10 mWycm2.
Fig. 5. Fill factor of ITOyPEDOT:PSSyOC1 C10 -PPV:PCBMyAl device as a function of light intensity at temperatures between 120 and 320 K.
3.2.2. Fill factor and parallel resistance Fig. 5 shows the FF as a function of illumination intensity. Above Ts100–140 K, the FF depends on the light intensity very weakly, i.e. varies within 6%, as compared to 28% at Ts120 K. As expected, the highest FF of 51% is measured at the highest temperature applied Ts320 K. The FF depends on both, series resistance, RS, and parallel resistance, RP, in a complex way. Most dramatic is the variation of RP with light intensity, as shown in Fig. 6. The RP, which is in the order of 100 kVØcm2 at a light intensity of 0.03 mWy cm2, decreases by nearly three orders of magnitude at 100 mWycm2. This is qualitatively in agreement with the degradation of FF at higher illumination levels. In contrast, the RS (not shown) varies very little, from 3 VØcm2 at PLights0.03 mWycm2 to 2.8 VØcm2 at 100 mWycm2, both at Ts320 K. The light intensity depend-
Fig. 6. Parallel resistance RP as a function of light intensity in log– log scale at temperatures between 120 and 320 K.
V. Dyakonov / Thin Solid Films 451 – 452 (2004) 493–497
ent RP is an important factor to be taken into account when applying the Shockley equation for the fitting of J–V characteristics under illumination in this type of solar cells. 4. Conclusions The parameters of ITOyPEDOT:PSSyOC1 C10 -PPV: PCBMyAl solar cells exhibit a strong dependence on light intensity as well as on temperature. Already at room temperature, the open circuit voltage exceeds the value expected from the work function difference of the electrode materials used by 100 mV. The VOC increases further upon cooling the device down to 120 K and, therefore, may be determined by HOMO-LUMO gap of the absorber. The VOC increases logarithmically with light intensity. However, at low temperatures, a saturation of the VOC at higher intensities is observed and remains to be clarified. The short circuit photocurrent is strongly temperature dependent and has a value of 4.5 mAycm2 at Ts320 K. It is thermally activated due to the trapping of photogenerated charge carriers by shallow traps. The activation energy (trap depth) is determined as 45 meV and is independent of light intensity. The dominant contribution of shallow defects to the temperature dependence of JSC rather than that of the mobility is supported by the monomolecular recombination of photogenerated charges observed, whereby the short circuit current varies linearly with light intensity in the whole temperature range. The overall solar cell efficiency strongly increases with the temperature and is only weakly dependent on the light intensity, reaching the maximum value of 1.9% at Ts320 K and 100 mWycm2 (white light) illumination. The absence of bimolecular recombination, being the intrinsic feature of polymer-fullerene networks, together with a strong temperature dependence of the short circuit current, confirm that the efficiency of ITOy PEDOT:PSSyOC1C10-PPV:PCBMyAl devices is limited by transport properties of the absorber. As a result, the active layer thickness must be kept low at the expense of the photogeneration of charge carriers.
497
Acknowledgments This work was financially supported by the European Commission (Project HPRN-CT-2000-00127) and by the BMBF (German Ministry for Education and Research, BMBF) (Project 01SF0026). Many thanks go to the following persons for their contribution in various discussions and assistance: J. Parisi, E.L. Frankevich, I. Riedel, Z. Chiguvare, E. von Hauff, D. Chirvase, M. Pientka, and H. Koch (all University of Oldenburg), J.C. Hummelen (U Groningen, NL), L. Lutsen and D. Vanderzande (IMOMEC, LUC, Belgium). References w1x H.S. Nalwa (Ed.), Handbook of Organic Conductive Molecules and Polymers, 1–4, John-Wiley and Sons, New York, 1997. w2x S. Karg, W. Rieß, V. Dyakonov, M. Schwoerer, Synth. Met. 54 (1993) 427. w3x G. Yu, J. Gao, J.C. Hummelen, F. Wudl, A.J. Heeger, Science 270 (1995) 1789. w4x P. Peumans, A. Yakimov, S.R. Forrest, J. Appl. Phys. 93 (2003) 3693. w5x (a) S.E. Shaheen, C.J. Brabec, F. Padinger, T. Fromherz, J.C. Hummelen, N.S. Sariciftci, Appl. Phys. Lett. 78 (2001) 841 (b) C.J. Brabec, N.S. Sariciftci, J.C. Hummelen, Adv. Funct. Mater. 11 (2001) 15. w6x (a) L.J. Lutsen, A.J. Van Breemen, W. Kreuder, D.J.M. Vanderzande, J.M.J.V. Gelan, Helv. Chim. Acta 83 (2000) 3113 (b) L.J. Lutsen, P. Adriaensens, H. Becker, A.J. Van Breemen, D. Vanderzande, J. Gelan, Macromolecules 32 (1999) 6517. w7x H. Antoniadis, L.J. Rothberg, F. Papadimitrakopoulos, M. Yan, M.E. Galvin, Phys. Rev. B 50 (1994) 14 911. w8x C.J. Brabec, A. Cravino, D. Meissner, N.S. Sariciftci, T. Fromherz, M.T. Rispens, L. Sanches, J.C. Hummelen, Adv. Funct. Mater. 11 (2001) 374. w9x I.D. Parker, J. Appl. Phys. 75 (1994) 1656. w10x (a) B.A. Gregg, M.C. Hanna, J. Appl. Phys. 93 (2003) 3605 (b) J.A. Barker, C.M. Ramsdale, N.C. Greenham, Phys. Rev. B 67 (2003) 075205 ¨ (c) P. Wurfel, G.-H. Bauer, in: C.J. Brabec, V. Dyakonov, J. Parisi, N.S. Sariciftci (Eds.), Organic Photovoltaics, Springer, Heidelberg, 2002, Ch. 4. w11x R.H. Bube, Photoelectronic Properties of Semiconductors, University Press, Cambridge, UK, 1992. w12x (a) V. Dyakonov, Physica E 14 (2002) 53 (b) V. Dyakonov, I. Riedel, C. Deibel, J. Parisi, C.J. Brabec, N.S. Sariciftci, J.C. Hummelen, Mater. Res. Soc. Symp. Proc. 605 (2001)(c) I. Riedel, J. Parisi, V. Dyakonov, F. Giacalone, N. Martin (this volume). w13x C.M. Ramsdale, J.A. Barker, A.C. Arias, J.D. MacKenzie, R.H. Friend, N.C. Greenham, J. Appl. Phys. 92 (2002) 4266.
Electrical aspects of operation of polymer–fullerene solar cells Vladimir Dyakonov* Energy and Semiconductor Research Laboratory, Institute of Physics, University of Oldenburg, D-26111 Oldenburg, Germany
Abstract Easily processable conjugated polymers for optoelectronics, in general, and for photovoltaic energy conversion, in particular, is an attractive research field, in which the combined efforts of material science, device engineers, and spectroscopists are welcomed. To improve the efficiency of polymer solar cells, it is vital to understand which mechanisms control the current–voltage characteristics of a given device. The current–voltage characteristics of ITOyPEDOT:PSSyOC1 C10 -PPV:PCBMyAl solar cells were measured in the temperature range 120–320 K under variable illumination, between 0.03 and 100 mWycm2 (white light). The short circuit current density grows monotonically with temperature until 320 K. This is indicative of a thermally activated transport of photogenerated charge carriers, influenced by recombination with shallow traps. A gradual increase of the open circuit voltage to 0.91 V was observed upon cooling the devices down to 120 K. The overall effect of temperature on solar cell parameters results in a positive temperature coefficient of the power conversion efficiency, which is 1.9% at Ts320 K and 100 mWycm2. The nearly linear variation of the short circuit current density with light intensity confirms that the internal recombination losses are predominantly of a monomolecular type under short circuit conditions. 䊚 2003 Elsevier B.V. All rights reserved. PACS: 72.20.Jv; 72.40.qw; 72.80.Le; 73.40.Sx; 73.61.Ph; 85.30.De Keywords: Polymer semiconductors; Photovoltaic effect; Charge carriers trapping
1. Introduction The use of conjugated polymers and molecules exhibits a high potential for production of efficient, and at the same time, low-cost, flexible optoelectronic devices with the option for large area applications w1x. The expanding field of organic electronic devices has stimulated a lot of attention and research on fundamental questions concerning the transport properties of electrically conducting polymers as well as the metalyorganic semiconductor interfaces. For photovoltaic applications, high photogeneration yield (generation of electrons and holes due to light absorption) is important w2x. One approach to achieve an efficient charge carrier generation in polymer light absorbers is to blend them with suited acceptors. For example, in polymer–fullerene composites, the conjugated polymer acts as a donor, and the fullerene molecule as an acceptor. One of the most effective and therefore extensively studied device concepts, so far, is based on the bulk heterojunction approach w3,4x. In this, the fullerene molecules are *Tel.: q49-441-7983538; fax: q49-441-7983326. E-mail address: [email protected] (V. Dyakonov).
dispersed in a polymer or in a low molecular weight organic matrix. The thin photoactive film is then sandwiched between two electrodes with asymmetric work functions. In this configuration, each electrode is expected to form an ohmic contact with the respective p-, or n- type semiconductor, while simultaneously blocking the minority charge carriers. For single junction solar cells based on the polymer bulk heterojunction concept, a power conversion efficiency of 2.5% has been achieved under 1 sun AM1.5 illumination conditions without light trapping w5x. This value is still quite modest for applications. However, much higher development potential can be anticipated taking the high quantum yield of charge carrier generation in these composites into account. In spite of significant advances in the understanding of the qualitative behaviour of organic solar cells, a fully quantitative description of charge injection at the electrodes, charge transport in the bulk, and electron-hole recombination has yet to emerge. This indicates the importance of the electrical transport properties of the absorber material and the interfaces. In order to further develop the bulk heterojunction concept, as well as to identify factors limiting the solar cell’s
0040-6090/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2003.11.063
494
V. Dyakonov / Thin Solid Films 451 – 452 (2004) 493–497
performance, a detailed understanding of the underlying device physics is imperative. We studied the influence of temperature and illumination intensity on the principal parameters of the polymer-fullerene bulk heterojunction solar cells, such as short circuit current and open circuit voltage. 2. Experimental The devices studied in this work were made in a ITOyPEDOT:PSSyOC1 C10 -PPV:PCBMyAl configuration. The active layer was made by spin coating a 1:4 chrolobenzene solution of poly w2-methoxy-5- (3-,7dimethyl-octyloxy)-1,4-phenylene vinylenex (OC1C10PPV) and phenyl-C61 butyric acid methyl ester (PCBM) onto a conducting substrate. The OC1C10-PPV was synthesized via the sulfinyl route as described elsewhere w6x. As a hole extracting electrode, the 80-nm layer of poly-wethylene dioxy thiophenex doped with polystyrene sulfonate (PEDOT: PSS), BAYTRON P, (Bayer AG, Germany) was spin coated on a glass substrate, coated with patterned indium tin oxide. The aluminium counter electrode was ther˚ mally evaporated in a vacuum chamber at rate 4 Ays. The current–voltage measurements were carried out in a variable temperature cryostat equipped with a liquid nitrogen supply and a temperature controller (Lakeshore 330) with a source-meter unit (Avantest TR-6143). The devices were illuminated through a sapphire window by white light from a xenon arc lamp whose intensity is controlled by a set of neutral density filters with transmission coefficients from 10y4 to 1. The maximum intensity was calibrated to 100 mWycm2 inside the cryostat. 3. Results and discussion Current density–voltage (J–V) measurements were performed in the temperature range Ts120–320 K in the dark, and under illumination for incident light intensities PLight from 0.03 to 100 mWycm2. The solar cell parameters, i.e. the open circuit voltage VOC, the short circuit current density JSC, the fill factor FF, and the power conversion efficiency h were determined. 3.1. Temperature dependence 3.1.1. Short circuit current Fig. 1 shows the short circuit current density JSC vs. temperature under different illumination intensities. The short circuit current density of the studied samples increases with temperature. The strong temperature effect we observed may be due to the electronic transport properties of the absorber material, e.g. due to variation of mobility. Further, it is negatively influenced by the capture of charge carriers by traps w7x. In both cases,
Fig. 1. Short circuit current density JSC as a function of temperature at light intensities (white light) indicated in the legend. Dashed lines represent calculated values with Eq. (1). For all light intensities we obtained the activation energy of Ds45 meV. Active layer thickness ds80 nm. Polymer–Fullerene weight ratio is 1:4.
one expects that an increase in temperature will promote the current through the device. In the absence of monoor bimolecular recombination, the JSC is expected to be weakly temperature dependent, mainly due to thermal generation of charge carriers across the semiconductor band gap. In this case, the thermally activated variation of mobility should not influence the steady state photogenerated current. The total amount of photogenerated charges will remain unaffected. Instead, the capture and emission of charge carriers by shallow traps may lead to a variation of the current with temperature. This is a thermally activated process and is denoted as monomolecular recombination. To prove this, the experimental data in Fig. 1 were fitted with the expression: B
JSCŽT,PLight.sJ0ŽPLight.expCy D
DE F, kT G
(1)
where J0(PLight) is a pre-exponential factor, which is comprised of the thermally generated charge carrier density, their mobility, and the electric field. Finally, D is the trap depth with respect to the corresponding band. From the fits we obtained the activation energy Ds45 meV for all the light intensities applied, and a strictly linear dependence for the pre-exponential factor with light intensity (not shown). We note that this interpretation is only valid in case of monomolecular recombination of photogenerated charges. In case of bimolecular recombination, e.g. due to the space charge effect, the thermally activated mobility will be responsible for the temperature dependence of the short circuit current, provided the traps are filled. The occurrence of bimolecular recombination can be established in a square root current density–intensity relationship.
V. Dyakonov / Thin Solid Films 451 – 452 (2004) 493–497
495
is the acceptor density, m is the mobility. Therefore, VOCs
Fig. 2. Open circuit voltage VOC as a function of temperature under different illumination intensities (white light) as indicated in the legend.
3.1.2. Open circuit voltage The origin of the open circuit voltage in bulkheterojunction solar cells is a matter of discussions. Physically the VOC is controlled by the splitting of quasiFermi levels in the absorber. It was found to correlate directly with the acceptor strength, i.e. it is determined by the HOMO-LUMO energy gap of donor and acceptor, respectively, being nearly independent of electrode materials w8x. In a metal-insulator-metal (MIM) picture w9x, the VOC is limited by the work function difference between electrodes, i.e. is equal to the built-in field. In Ref. w10x, the VOC is expected to be determined by both the built-in field and the photoinduced contribution, i.e. by the electrochemical potential gradient across the absorber layer. The open circuit voltage was measured as function of temperature at different illumination intensities and is shown in Fig. 2. At Ts300 K and the highest illumination intensity used, PLights100 mWycm2, the VOC is 0.82 V. Upon cooling down the sample to Ts120 K, the VOC increases to 0.91 V. For conventional inorganic solar cells an increase in VOC over a whole temperature range is expected w11x. The temperature dependence of VOC is based on Shockley equation for an illuminated p–n solar cell: w
B
y
D
JsJ0xexpCy
qV E z Fy1|yJL, kT G ~
E kT B JSC Eq kT B NVNCmkT E F. lnC q1Ff y lnC G q D J0 q q D JSCLNA G
(4)
In limit T™0 K, the VOC approaches Eg yq, where Eg is the band gap energy of the absorber material, q is the elementary charge. The applicability of such an approach to polymer solar cells is unclear, as the temperature dependence of several important parameters, i.e. of diffusion length and acceptor density, is unknown. However, one can see the tendency of curves to converge at low temperatures. Already the room temperature value is higher than the one expected from MIM picture, i.e. 0.7 V for Al and PEDOT:PSS, and seems to be determined andyor limited by the energetic gap between the polymer’s HOMO and the LUMO of the acceptor material, 1.2 eV. This feature of polymer solar cells was observed for many different composites used in the absorber, in some cases showing VOC well above the difference in the work functions of the electrodes, already at room temperature w12,13x. 3.2. Illumination intensity dependence The charge carriers in the solar cells under investigation are efficiently photogenerated via ultrafast electron transfer between donor and acceptor counterparts of the composite, usually having a very low charge carrier concentration in the dark. The fate of the photogenerated electrons and holes is crucial for the device efficiency, therefore, the influence of light intensity on the J–V characteristics is important. 3.2.1. Short circuit current and open circuit voltage Fig. 3 displays the short circuit current density as function of light intensity PLight in a double-logarithmic
(2)
where JL is photogenerated current density, JsJLfJSC at Vs0, and J0 is saturation current density: B yE E
J0sNVNCmkTexpC D
1 , kT G LNA g
F
(3)
where NVyC is the effective density of states in the valenceyconduction band, L is the diffusion length, NA
Fig. 3. Short circuit current density as a function of light intensity for different temperatures. The scaling exponents (JSCAPaLight) between 0.92 and 0.85 are obtained for Ts320 K and Ts120 K, respectively.
496
V. Dyakonov / Thin Solid Films 451 – 452 (2004) 493–497
Fig. 4. Open circuit voltage as a function of light intensity in a semilog scale at temperatures between 120 and 320 K.
scale. The JSC follows the power law dependence JSCs PaLight with scaling exponents as0.92 and 0.85 for Ts 320 K and Ts120 K, respectively. The nearly linear dependence of the JSC indicates that charge carrier losses in the absorber bulk are dominated by the monomolecular recombination in defects or impurities. The bimolecular e-h recombination was not observed even at highest illumination intensity. This is in agreement with the picture of photogeneration and transport of charge carriers in a donor–acceptor bulk heterojunction: an ultrafast e-h separation is followed by the transport of negatively and positively charged carriers within spatially separated networks: electrons along percolated fullerenes travel towards the Al contact, holes within the polymer matrix travel towards the PEDOT:PSS contact. As both contacts may be considered ohmic for the respective charge carriers (i.e. the potential barriers are below 0.5 eV), no space charge limited conditions under short circuit conditions, even at highest generation rates, exist. The absence of bimolecular recombination together with a strong temperature dependence of the short circuit current confirm our assumption that the efficiency of devices is limited by the charge transport properties of the absorber materials, i.e. by recombination with shallow traps. We note that this is valid by assuming a temperature independent charge carrier generation rate. Provided the short circuit current density is proportional to the light intensity (see Fig. 3) and the saturation current density J0 is independent of the illumination level, the VOC is expected to be proportional to ln(JSC). This behaviour (Fig. 4) was found in the higher temperature range, Ts280–320 K, only, whereas at lower temperatures all curves saturate at intensities between 1 and 10 mWycm2.
Fig. 5. Fill factor of ITOyPEDOT:PSSyOC1 C10 -PPV:PCBMyAl device as a function of light intensity at temperatures between 120 and 320 K.
3.2.2. Fill factor and parallel resistance Fig. 5 shows the FF as a function of illumination intensity. Above Ts100–140 K, the FF depends on the light intensity very weakly, i.e. varies within 6%, as compared to 28% at Ts120 K. As expected, the highest FF of 51% is measured at the highest temperature applied Ts320 K. The FF depends on both, series resistance, RS, and parallel resistance, RP, in a complex way. Most dramatic is the variation of RP with light intensity, as shown in Fig. 6. The RP, which is in the order of 100 kVØcm2 at a light intensity of 0.03 mWy cm2, decreases by nearly three orders of magnitude at 100 mWycm2. This is qualitatively in agreement with the degradation of FF at higher illumination levels. In contrast, the RS (not shown) varies very little, from 3 VØcm2 at PLights0.03 mWycm2 to 2.8 VØcm2 at 100 mWycm2, both at Ts320 K. The light intensity depend-
Fig. 6. Parallel resistance RP as a function of light intensity in log– log scale at temperatures between 120 and 320 K.
V. Dyakonov / Thin Solid Films 451 – 452 (2004) 493–497
ent RP is an important factor to be taken into account when applying the Shockley equation for the fitting of J–V characteristics under illumination in this type of solar cells. 4. Conclusions The parameters of ITOyPEDOT:PSSyOC1 C10 -PPV: PCBMyAl solar cells exhibit a strong dependence on light intensity as well as on temperature. Already at room temperature, the open circuit voltage exceeds the value expected from the work function difference of the electrode materials used by 100 mV. The VOC increases further upon cooling the device down to 120 K and, therefore, may be determined by HOMO-LUMO gap of the absorber. The VOC increases logarithmically with light intensity. However, at low temperatures, a saturation of the VOC at higher intensities is observed and remains to be clarified. The short circuit photocurrent is strongly temperature dependent and has a value of 4.5 mAycm2 at Ts320 K. It is thermally activated due to the trapping of photogenerated charge carriers by shallow traps. The activation energy (trap depth) is determined as 45 meV and is independent of light intensity. The dominant contribution of shallow defects to the temperature dependence of JSC rather than that of the mobility is supported by the monomolecular recombination of photogenerated charges observed, whereby the short circuit current varies linearly with light intensity in the whole temperature range. The overall solar cell efficiency strongly increases with the temperature and is only weakly dependent on the light intensity, reaching the maximum value of 1.9% at Ts320 K and 100 mWycm2 (white light) illumination. The absence of bimolecular recombination, being the intrinsic feature of polymer-fullerene networks, together with a strong temperature dependence of the short circuit current, confirm that the efficiency of ITOy PEDOT:PSSyOC1C10-PPV:PCBMyAl devices is limited by transport properties of the absorber. As a result, the active layer thickness must be kept low at the expense of the photogeneration of charge carriers.
497
Acknowledgments This work was financially supported by the European Commission (Project HPRN-CT-2000-00127) and by the BMBF (German Ministry for Education and Research, BMBF) (Project 01SF0026). Many thanks go to the following persons for their contribution in various discussions and assistance: J. Parisi, E.L. Frankevich, I. Riedel, Z. Chiguvare, E. von Hauff, D. Chirvase, M. Pientka, and H. Koch (all University of Oldenburg), J.C. Hummelen (U Groningen, NL), L. Lutsen and D. Vanderzande (IMOMEC, LUC, Belgium). References w1x H.S. Nalwa (Ed.), Handbook of Organic Conductive Molecules and Polymers, 1–4, John-Wiley and Sons, New York, 1997. w2x S. Karg, W. Rieß, V. Dyakonov, M. Schwoerer, Synth. Met. 54 (1993) 427. w3x G. Yu, J. Gao, J.C. Hummelen, F. Wudl, A.J. Heeger, Science 270 (1995) 1789. w4x P. Peumans, A. Yakimov, S.R. Forrest, J. Appl. Phys. 93 (2003) 3693. w5x (a) S.E. Shaheen, C.J. Brabec, F. Padinger, T. Fromherz, J.C. Hummelen, N.S. Sariciftci, Appl. Phys. Lett. 78 (2001) 841 (b) C.J. Brabec, N.S. Sariciftci, J.C. Hummelen, Adv. Funct. Mater. 11 (2001) 15. w6x (a) L.J. Lutsen, A.J. Van Breemen, W. Kreuder, D.J.M. Vanderzande, J.M.J.V. Gelan, Helv. Chim. Acta 83 (2000) 3113 (b) L.J. Lutsen, P. Adriaensens, H. Becker, A.J. Van Breemen, D. Vanderzande, J. Gelan, Macromolecules 32 (1999) 6517. w7x H. Antoniadis, L.J. Rothberg, F. Papadimitrakopoulos, M. Yan, M.E. Galvin, Phys. Rev. B 50 (1994) 14 911. w8x C.J. Brabec, A. Cravino, D. Meissner, N.S. Sariciftci, T. Fromherz, M.T. Rispens, L. Sanches, J.C. Hummelen, Adv. Funct. Mater. 11 (2001) 374. w9x I.D. Parker, J. Appl. Phys. 75 (1994) 1656. w10x (a) B.A. Gregg, M.C. Hanna, J. Appl. Phys. 93 (2003) 3605 (b) J.A. Barker, C.M. Ramsdale, N.C. Greenham, Phys. Rev. B 67 (2003) 075205 ¨ (c) P. Wurfel, G.-H. Bauer, in: C.J. Brabec, V. Dyakonov, J. Parisi, N.S. Sariciftci (Eds.), Organic Photovoltaics, Springer, Heidelberg, 2002, Ch. 4. w11x R.H. Bube, Photoelectronic Properties of Semiconductors, University Press, Cambridge, UK, 1992. w12x (a) V. Dyakonov, Physica E 14 (2002) 53 (b) V. Dyakonov, I. Riedel, C. Deibel, J. Parisi, C.J. Brabec, N.S. Sariciftci, J.C. Hummelen, Mater. Res. Soc. Symp. Proc. 605 (2001)(c) I. Riedel, J. Parisi, V. Dyakonov, F. Giacalone, N. Martin (this volume). w13x C.M. Ramsdale, J.A. Barker, A.C. Arias, J.D. MacKenzie, R.H. Friend, N.C. Greenham, J. Appl. Phys. 92 (2002) 4266.