PCCP View Article Online Published on 12 December 2012. Downloaded by University of Oxford on 08/06/2014 16:40:01. PAPER Cite this: Phys. Chem. Chem. Phys., 2013, 15, 2572 View Journal | View Issue Lithium salts as ‘‘redox active’’ p-type dopants for organic semiconductors and their impact in solid-state dye-sensitized solar cells† Antonio Abate,za Tomas Leijtens,za Sandeep Pathak,a Joël Teuscher,a Roberto Avolio,b Maria E. Errico,b James Kirkpatrik,a James M. Ball,a Pablo Docampo,a Ian McPhersonc and Henry J. Snaith*a Lithium salts have been shown to dramatically increase the conductivity in a broad range of polymeric and small molecule organic semiconductors (OSs). Here we demonstrate and identify the mechanism by which Li+ p-dopes OSs in the presence of oxygen. After we established the lithium doping mechanism, we re-evaluate Received 6th December 2012, Accepted 11th December 2012 the role of lithium bis(trifluoromethylsulfonyl)-imide (Li-TFSI) in 2,2 0 ,7,7 0 -tetrakis(N,N-di-p-methoxyphenylamine)9,9 0 -Spirobifluorene (Spiro-OMeTAD) based solid-state dye-sensitized solar cells (ss-DSSCs). The doping DOI: 10.1039/c2cp44397j mechanism consumes Li+ during the device operation, which poses a problem, since the lithium salt is required at the dye-sensitized heterojunction to enhance charge generation. This compromise highlights www.rsc.org/pccp that new additives are required to maximize the performance and the long-term stability of ss-DSSCs. Introduction Organic semiconductors (OSs) are currently considered for a wide range of applications, including light emitting diodes,1 sensors,2 field effect transistors,3 and solar cells.4–6 Their solution-processable manufacturing, by means of versatile printing techniques, provides a large potential for the future electronic market. One of the original scientific breakthroughs for organic solids was the ability to modulate their electrical conductivity using chemical doping, and hence demonstrate their semiconducting properties.7 Similar to inorganic semiconductors, doping intentionally introduces chemical impurities into the OSs for the purpose of adjusting material properties to the specific application; e.g., triphenylamine-based OSs have a Clarendon Laboratory, Department of Physics, University of Oxford, Parks Road, Oxford, OX1 3PU, UK. E-mail: h.snaith1@physics.ox.ac.uk b Institute of Polymer Chemistry and Technology (ICTP), National Research Council of Italy, Via Campi Flegrei 34, 80078 Pozzuoli (NA), Italy c Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, South Parks Road, Oxford, OX1 3QR, UK † Electronic supplementary information (ESI) available: Estimation of effective series resistance due to the hole transporter. Ideal diode fit. Estimation of power losses through the devices due to series resistance. Estimation of possible conductivities of hole transporter (Fig. S1). Metal-semiconductor contact resistance (Fig. S2, Table S1). IR and UV-Vis measurements. Solid state 7Li NMR (Fig. S3). Ground state dye absorption (Fig. S4). Spiro-OMeTAD on SiO2 conductivity in dry O2. See DOI: 10.1039/c2cp44397j ‡ These authors contributed equally. 2572 Phys. Chem. Chem. Phys., 2013, 15, 2572--2579 been p-doped with Co(III) complexes in solid-state dye-sensitized solar cells (ss-DSSCs),8 and tetracyano-quinoline derivatives in organic light emitting diodes (OLEDs).9 Both applications benefit from the introduction of a p-doped transport layer since it reduces the charge transport resistance in series with the heterojunction8,10–12 and helps to achieve Ohmic contacts.13–15 Generally, chemical p-doping agents tend to be comprised of molecules with relatively high electron affinities, often in the form of metal–organic complexes or organic resonance structures, which can generate unpaired electrons in the organic matrix by means of stable charge transfer complexes.16–19 The overall result of such doping is a significant increase in the concentration of free holes in the OS valence band or highest occupied molecular orbital (HOMO) levels, with the associated enhancement in conductivity, especially at high concentrations.20,21 Recently, lithium salts have been reported to enhance the conductivity in a broad range of polymeric22,23 and small molecule24,25 OSs. Yanagida et al. found that lithium salts enhance the hole transport in polythiophene derivatives.26 They discussed the effect of different anions without considering the role of the Li+ ion itself. Very similar properties have been described for lithium bis(trifluoromethylsulfonyl)-imide (Li-TFSI) in polyphenylenevinylene derivatives.27,28 Li-TFSI has also been shown to dramatically increase the conductivity and hole mobility in small molecule OSs, such as 2,2 0 ,7,7 0 -tetrakis(N,Ndi-p-methoxyphenyl-amine)9,90 -Spirobifluorene (Spiro-OMeTAD).29 This journal is c the Owner Societies 2013 View Article Online Published on 12 December 2012. Downloaded by University of Oxford on 08/06/2014 16:40:01. Paper PCCP To explain the effect of increased conductivity and apparent increase in mobility, it was previously suggested that the ionic species in the organic matrix smooth the potential landscape, which acts to increase the probability of intra-molecular charge transfer.20,29,30 The resultant 5–10 fold increase in hole mobility was then used to rationalize the observed 100 fold increase in conductivity. The Li-TFSI additive was considered redox inactive, and initial UV-Vis absorption measurements showed no signature of oxidized hole transporter in solution.29 For this reason, the hole density was not considered to be significantly affected by addition of Li-TFSI, leaving at least an order of magnitude increase in conductivity unaccounted for. ss-DSSCs incorporating molecular or polymeric holetransporters have also employed lithium salts as additives to the organic phase, since their incarnation.6,31 The primary role is to force a negative shift (further from vacuum) in the surface potential of the TiO2, favouring forward electron-transfer from the photoexcited tethered dye.32 However, the influence of the lithium salts in the ss-DSSCs is broad ranging.14,29,32–38 and despite many studies on the subject a large degree of ‘‘black magic’’ seems to prevail. Very recently, Cappel et al. reported that p-doping the hole transporter is fundamental for devices operation and lithium salts could catalyse the hole transporter oxidation (p-doping), which takes place at TiO2/dye interface under illumination.39 The present study discusses the mechanism that drives the lithium salts to behave as effective p-dopants in a broad range of polymeric and small molecule OSs. Contrary to previous reports,29 we show that Li+ is involved in redox activity, which leaves free holes in the organic matrix. This mechanism explains the dramatic increase in conductivity observed in many lithium salt–hole transporter systems.22,24–28 After we established the lithium doping mechanism, we investigate the role of the Li-TFSI in SpiroOMeTAD based ss-DSSCs, specifically looking at its influence on conductivity of the hole-transporter. We demonstrated the lithium doping of Spiro-OMeTAD, whereby Li+ is consumed in presence of oxygen, leading to stabilized p-doping which occurs regardless of the interface with TiO2 or dye molecules. This scenario helps explain the variation of device performance as function of the lithium content, presence of oxygen, and time. Experimental section ss-DSSCs were prepared according to a standard procedure,29 and all solvents used in this work were reagent grade and anhydrous. Briefly, FTO substrates (15 O cm1, Pilkington) were etched with zinc powder and HCl (2 M) to give the desired electrode patterning. The substrates were cleaned with Hellmanex (2% in de-ionized water), de-ionized water, acetone, and ethanol. The last traces of organic residues were removed by a 10 min oxygen plasma cleaning step. The FTO sheets were subsequently coated with a compact layer of TiO2 (100 nm) by aerosol spray pyrolysis deposition at 300 1C, using air as the gas carrier. 1 mm thick mesoporous TiO2 films were then deposited by screenprinting a commercial paste (Dyesol 18NR-T). The TiO2 films were slowly heated to 500 1C and allowed to sinter for 30 min in air. This journal is c the Owner Societies 2013 The samples were immersed into 0.015 M TiCl4 aqueous solution for 45 min at 70 1C, and then heated to 500 1C for another sintering step of 45 min. After cooling to 70 1C, the substrates were immersed in a 0.5 mM ((5-{1,2,3,3a,4,8bhexahydro-4-[4-(2,2-diphenylvinyl)phenyl]-cyclopenta[b]indole7-ylmethylene}-4-oxo-2-thioxo-thiazolidin-3-yl)acitic acid) (D102), dye solution (in 1 : 1 mixture of acetonitrile and tert-butyl alcohol) for one hour. After the dyed films were rinsed in acetonitrile, the Spiro-OMeTADhole conductor matrix was applied by spin coating at 1000 rpm for 60 s in air. The solutions for spin coating consisted of Spiro-OMeTAD dissolved in anhydrous chlorobenzene (reagent grade) at 15% v/v, assuming a density of Spiro-OMeTAD of 1 g cm3. This concentration gives 700 nm capping layer of Spiro-OMeTAD on top of the mesoporous TiO2 layer. tert-Butyl pyridine (tbp) was added to the solution at a concentration of 33 mol% relative to Spiro-OMeTAD. Lithium bis(trifluoromethylsulfonyl)imide salt (Li-TFSI) (170 mg ml1 in acetonitrile) was added to achieve the molar equivalent, relative to Spiro-OMeTAD listed in the main text. After drying overnight, back contacts were applied by thermal evaporation of 200 nm of silver. For measuring the device merit parameters, Solar-simulated AM 1.5 sunlight was generated with an ABET solar simulator calibrated to give 100 mW cm2 using an NREL-calibrated KG5 filtered silicon reference cell, and the I/V curves were recorded with a sourcemeter (Keithley 2400, USA). The solar cells were masked with a metal aperture defining the active area (0.09 cm2) of the solar cells.40 The photovoltage and photocurrent decay measurements were performed by a similar method to O’Regan et al. and as described elsewhere.41–43 All devices were stored in air (dark condition) for 72 hours before testing. Devices for measuring the conductivity of the hole conductor in a dye-sensitized mesoporous titania film were prepared according to a published procedure.29 The preparation (on glass substrates) was identical to that used for the ss-DSSCs, the only differences being that no compact layer of TiO2 was deposited, the Spiro-OMeTAD solution in chlorobenzene was 10% v/v to give a thinner capping layer, and the electrode pattern was designed for two point probe measurements with a channel length (direction of current flow) of 200 mm, a channel width of 6.53 cm, and a film thickness of 1 mm was used. The spin coating solutions again consisted of the same concentration of tbp but varying concentrations of Li-TFSI, as described in the main text. Linear I/V curves were obtained for the conductivity measurements by testing in ambient light conditions (no light dependence was noted) with a sourcemeter (Keithley 2400, USA) in a two-point contact setup. The contact resistance was several order of magnitude lower then bulk resistance in the semiconductor, as estimated by impedance spectroscopy (see ESI† for details). The devices for measurements in nitrogen atmosphere were prepared in a glovebox starting from Spiro-OMeTAD deposition, left for 48 hours, and tested in a nitrogen atmosphere. 7 Li solid state NMR spectra were recorded at 155.45 MHz on a Bruker Avance II spectrometer equipped with a 4 mm MAS probe. A p/2 pulse width of 4.5 ms was used, with a relaxation delay of 120 s and a spinning speed of 8–10 kHz. Chemical shifts are referenced to external LiCl (aq) 1 M. Phys. Chem. Chem. Phys., 2013, 15, 2572--2579 2573 View Article Online Published on 12 December 2012. Downloaded by University of Oxford on 08/06/2014 16:40:01. PCCP Paper IR spectra were collected on a Bio-Rad FTS-6000 FTIR spectrometer fitted with an MCT detector using a DuraSamplIR II diamond ATR accessory. Each spectrum is the result of 50 co-added scans at a resolution of 1 cm1 and are displayed as A = log10(R/R0) where R0 is a blank spectrum of the accessory. For the UV-Vis spectra, the sample preparation was identical to that described for the preparation of devices for conductivity measurements, with the exception that the electrodes were not sensitized by dye and that no counter-electrodes were deposited. Here the solutions of Spiro-OMeTAD were 10% v/v in anhydrous chlorobenzene, there was no added tbp, and the Li TFSI content was varied as described in the text. It should be noted that the samples were prepared and stored in a clean room under attenuated UV light. Absorbance measurements of the films were taken using a commercial spectrophotometer (Varian Cary 300 UV-Vis, USA) with an internally coupled integrating sphere (Labsphere, USA). Within the sphere is an integrated photomultiplier tube that detects the light. Samples were mounted at the entrance of the sphere, with a diffuse reflector mounted on an 81 wedge at the exit port. Baseline measurements were performed on mesoporous TiO2 samples on glass, the same as the substrates used for the samples. Results and discussion The first aim of this study is to understand the effect of lithium salts addition on the charge transport in OSs. To shed light on this phenomena, we focused the experiments on a model system using Li-TFSI, which ensures high electrochemical stability due to the TFSI anion,44 and Spiro-OMeTAD, which is the state-ofthe-art hole transporting material in ss-DSSCs. Fig. 1 displays the dependence of the Spiro-OMeTAD conductivity on the Li-TFSI content. We note that all conductivity values discussed in this work are ‘‘effective’’ conductivities through a mesoporous film sensitized with dye. This is the conductivity that is relevant to the solar cell. The method employed has been previously described for measuring the effective Spiro-OMeTAD conductivity in conditions similar to device operation.29 A strong trend is evident, where the conductivity increases from a very low 3 108 up to 3 105 S cm1. Notably, the conductivity reaches the maximum at the Li-TFSI content usually employed in ss-DSSCs (12–30 mol% with Spiro-OMeTAD),8 suggesting that this effect on the hole conduction may be important in the device performance. So far, this remarkable increase in conductivity has been exclusively ascribed to the electrostatic charge of the ionic species added into the organic matrix.26–29 In particular, it was claimed that the addition of Li-TFSI does not induce charge transfer complexes, and extra addition of chemical p-dopants is necessary to oxidize the Spiro-OMeTAD.8,29 However, the UV-Vis absorption spectra of mesoporous TiO2 films infiltrated with Spiro-OMeTAD (Fig. 1) show the clear growth of an absorption band around 520 nm as the concentration of Li-TFSI is increased, which is indicative of a Spiro-OMeTAD oxidized state (p-doping).45 The absorption increment with the Li-TFSI content suggests that the salt could be involved in the oxidation of Spiro-OMeTAD. In a very recent study, Cappel et al. for the first 2574 Phys. Chem. Chem. Phys., 2013, 15, 2572--2579 Fig. 1 Effective Spiro-OMeTAD conductivity and UV-Vis absorption spectra (inset) as function of the Li-TFSI content. These films were left for 78 hours in air before measurement. The solid-line is simply to aid the eye. time report the observation of oxidized Spiro-OMeTAD (p-doping) when Li-TFSI is added to the hole transporter in ss-DSSCs.39 They suggest that the Li+ may catalyse the photo-oxidation of the SpiroOMeTAD, which takes place on dye-sensitized TiO2 surface under illumination. However, we observe p-doping regardless the dye-sensitized TiO2 and light exposure (see ESI†), which suggests a different and more general doping mechanism from the proposed one.39 We have performed a time series of the conductivity measurements in different experimental conditions (atmosphere and salt content). In Fig. 2a we show the absorbance of the oxidized Spiro-OMeTAD probed at 520 nm as a function of time for a range of Li-TFSI contents. The rate of Spiro-OMeTAD oxidation seems to increase with Li-TFSI concentration, while the final degree of oxidation achieved scales approximately linearly with increasing Li-TFSI concentration. This allows us to conclude that Li-TFSI does not participate as a catalyst, but as a reagent being consumed while the reaction progresses.46 It is well known that in the primary processes of photooxidation, molecular oxygen plays the role of p-dopant.47–52 To probe whether oxygen is involved in the oxidation of SpiroOMeTAD, we prepared devices for measuring conductivity in oxygen-free conditions with and without the addition of a standard amount (10 mol% with Spiro-OMeTAD) of Li-TFSI. In Fig. 2b we show that for devices fabricated with and without Li-TFSI, but never exposed to air, the conductivity of SpiroOMeTAD is approximately the same. This observation confirms that the electrostatic effect of the ionic species29 alone is not sufficient to explain the increase in conductivity, and indeed raises the question as to whether the ‘‘smoothing of the potential landscape’’ from the presence of the TFSI counterion has a significant influence at all. Once exposed to air, the sample with Li-TFSI shows a rapid increase in conductivity of over two orders of magnitude to reach the same final conductivity value as previously determined for films made in air. The completion of this process for the same Li-TFSI content This journal is c the Owner Societies 2013 View Article Online Published on 12 December 2012. Downloaded by University of Oxford on 08/06/2014 16:40:01. Paper PCCP Fig. 2 Characteristics of Spiro-OMeTAD with different Li-TFSI content in air and in nitrogen atmosphere. (a) Variation of optical density probed at 520 nm in films as a function of time in air. The legend gives the mol% of Li-TFSI with respect to Spiro-OMeTAD. (b) The effective conductivity of Spiro-OMeTAD as a function of time, before (in nitrogen) and after exposure to air. The legend indicates neat Spiro-OMeTAD and with 10 mol% Li-TFSI. moreover seems to occur on a similar timescale as what is observed in Fig. 2a and is not reversible when the sample is placed back in oxygen-free conditions, since the conductivity remains at 8 106 S cm1 after 78 hours in a nitrogen environment. On the other hand, the sample without Li-TFSI (neat Spiro-OMeTAD) has a generally constant conductivity over time (the slight difference between nitrogen and air is discussed in ESI†), allowing us to conclude that both Li-TFSI and oxygen play a central role in the doping mechanism, and is distinct from the long-know oxygen doping mechanism.39,49–51,53 A possible contribute of the other air component (mainly water) was excluded performing a similar experiment in pure oxygen, as described in ESI.† In order to identify the reaction products, we have performed solid-state spectrochemical analyses on materials in conditions similar to device operation (see Experimental Section for details). In Fig. 3a we show the 7Li NMR of Li-TFSI and Spiro-OMeTAD with LiTFSI 10 mol% spectra recorded in the solid-state.54,55 At the concentration commonly employed in ss-DSSCs,8 Li-TFSI is perfectly solubilised in the Spiro-OMeTAD film as evidenced by relatively narrow lines under magic angle spinning, which suggest a weak homonuclear coupling.56 The spectrum of the doped Spiro-OMeTAD film, detected immediately after the preparation, contains a single 7Li resonance, slightly shifted from the bulk Li-TFSI signal due to the different chemical environment. Interestingly, after oxidation occurs (12 hours in air), two different 7Li resonances are recorded. This indicates a change in the coordination of Li atoms, with the formation of complexes57 which are This journal is c the Owner Societies 2013 Fig. 3 (a) Solid state 7Li NMR of neat Li-TFSI and Spiro-OMeTAD with the addition of 10 mol% Li-TFSI collected immediately after the preparation and after 12 h left in air (the neat Li-TFSI spectrum after 12 h in air is not reported being very similar to Li-TFSI in (a)). (b) ATR-FTIR spectra of the powder extracted from the oxidized Spiro-OMeTAD + Li-TFSI sample (which we postulate to contain LixOy), compared with the Li2O and Li2O2 (top graph) and the spectra of the starting material (bottom graph). likely to involve oxygen and the Spiro-OMeTAD molecules. In particular, the downfield peak (0.02 ppm) is shifted towards the range of lithium–oxygen species (3.3 ppm for Li2O, 0.3 ppm for Li2O2 as determined experimentally, see ESI†). The narrow line width indicates a molecular dispersion of these species rather than an organization in crystalline structures.58 In order to further investigate the reaction products, we dissolved the Spiro-OMeTAD Li-TFSI film in anhydrous chlorobenzene/ dimethyl sulfoxide to extract the formed oxides. When the solution is left in dark and nitrogen for few days, a white powder precipitates. We washed the filtered solid with anhydrous chlorobenzene and dried it in air. In Fig. 3b we show the ATR-FTIR spectra of the collected powder. The absorption profile is clearly different from the starting materials, and a large peak appears at 682 cm1. Similar peaks are observed in the IR spectra of Li2O and Li2O2 and can be ascribed to Li–O vibrational stretches.59 The data presented above allows us to draw qualitative conclusions concerning the mechanism that drives the lithium salts to behave as an effective p-dopant in the presence of oxygen, or otherwise put, enable oxygen to act as a stable p-dopant for organic semiconductors. The low ionization potential of Spiro-OMeTAD and the high electron affinity of the molecular oxygen leads to a weakly bound donor–acceptor complex, which can result in an effective electron transfer to the oxygen molecule through light or thermal excitation, as illustrated in eqn (1).60–63 As observed by the UV-Vis spectra and conductivity measurements, the concentration Phys. Chem. Chem. Phys., 2013, 15, 2572--2579 2575 View Article Online PCCP Paper of oxidized Spiro-OMeTAD (Spiro-OMeTAD+O2) is negligible in the absence of Li-TFSI. However, the addition of the Li+ changes the equilibrium of eqn (1) in the direction of the oxidized SpiroOMeTAD by consuming O2 as described in eqn (2).59,64–66 Spiro-OMeTAD + O2 2 Spiro-OMeTAD + O2 (1) Published on 12 December 2012. Downloaded by University of Oxford on 08/06/2014 16:40:01. Spiro-OMeTAD + O2 + Li-TFSI - Spiro-OMeTAD + TFSI + LixOy, (2) where LixOy stands for lithium oxide complexes, which we identify as Li2O or Li2O2 from the ATR-FTIR and 7Li NMR data presented in Fig. 3. The formed Spiro-OMeTAD radical cation (Spiro-OMeTAD+TFSI) is weakly bound by the highly delocalized charge on the TFSI anion,44 which results in an effective generation of mobile holes on the organic matrix. We note that according to the proposed mechanism, it is most likely that the oxygen radical will form LiO2 upon exposure to Li+ ions, which readily decomposes to Li2O2.59,64 In the remainder of the manuscript we determine the impact of the lithium doping on ss-DSSCs performance, by studying a series of devices with different Li-TFSI content. In Fig. 4 we show the device figures-of-merit for each Li-TFSI concentration.40 Confirming what was observed in the literature,33 there is a strong improvement in the short-circuit photocurrent (Jsc) and fill factor (FF), while the open circuit voltage (Voc) decreases with the increased Li-TFSI content. Notably, however, the short-circuit photocurrent begins to decrease as the Li-TFSI content is increased beyond 10% (molarity with respect to SpiroOMeTAD), although the FF continues to improve giving an optimal performance around 20 mol% Li-TFSI to Spiro-OMeTAD. This suggests that Li-TFSI has a broad impact on a number of processes occurring in the solar cells, which are possibly working in opposite senses. The ground state absorption in the dye-sensitizer (see ESI†) and the electron transfer efficiency from the photoexcited dye into Fig. 4 Devices performance parameters, efficiency (Z) short-circuit current (Jsc), opencircuit voltage (Voc) and fill factor (FF), as function of the Li-TFSI content, extracted from current voltage curves of solid-state DSCs employing D102 as the sensitizer measured under AM1.5 simulated sun light of 100 mW cm2 equivalent solar irradiance. Each point is representative of the four devices prepared per Li-TFSI content. 2576 Phys. Chem. Chem. Phys., 2013, 15, 2572--2579 Fig. 5 (a) Differential capacitance against voltage measurements for cells containing a range of Li-TFSI contents. (b) Transport lifetimes (ttrans) at short circuit conditions, (c) recombination lifetimes (trec) at open circuit conditions and (d) the charge collection efficiency (Zcol) as a function of charge density for the same range of Li-TFSI contents as in (a). The lifetime’s reported refer to the time constants obtained by fitting the current and voltage decays to mono-exponential decay functions. the TiO2 are strongly influenced by the presence of the Li-TFSI. This has been observed before for this specific dye, and has been assigned to a change in the local polarity of the hole-transporter medium in the vicinity of the dye.31,32 Additionally, the Li-TFSI also affects the charge dynamics at the TiO2–dye–Spiro-OMeTAD interface as well as electron and hole transport in the TiO2 bulk and Spiro-OMeTAD respectively.34–36 Disentangling the compounding effects of the lithium salts on the different interfaces and specifically its influence on transport is challenging. However, to quantify the influence of Li-TFSI in more detail, we have performed transient photo-voltage and photo-current decay measurements on ss-DSSCs with different Li-TFSI content. From these transient measurements, it is possible to extract estimates for the density of sub band gap states in the TiO2, recombination lifetime, charge collection lifetime and charge collection efficiency.67 For the devices studied here, we find that the density of states in the TiO2 (DOS, Fig. 5a) becomes deeper and broader as Li-TFSI is added, which is consistent with Bai et al.,37 and is attributed to a downwards (further from vacuum) shift in TiO2 conduction band potential. The slight broadening of the DOS has also been explained as the result of an increased number of trap states due to Li+ intercalation into the TiO2.67,68 The trend observed for the DOS fits with the lowering in Voc observed by increasing the lithium content while maintaining a constant concentration of tert-butyl pyridine (tBP) (see Experimental Section for details). In Fig. 5b we show the charge transport lifetime measured at short-circuit in the devices, which we observe to decreases with increasing Li-TFSI content, in contrast to the trend in hole-conductivity in the Spiro-OMeTAD. This is not surprising, since Fabregat-Santiago et al.38 have established by impedance modelling that at the low charge densities observed at short-circuit conditions, the slow multitrapping electron transport processes in TiO2 limits the charge collection rate from the device.38 Fabregat-Santiago et al. have This journal is c the Owner Societies 2013 View Article Online Published on 12 December 2012. Downloaded by University of Oxford on 08/06/2014 16:40:01. Paper PCCP also shown that the addition of Li-TFSI slows down the transport through TiO2 due to the addition of surface traps by lithium intercalation and hence reduces the detrapping rate and lowers the electron diffusivity in the TiO2.69 This is consistent with our observations here, where a broader density of sub band gap states has increased the average trap depth at any given charge density, and not just induced a shift in surface potential. At first sight however, this trend of reduced transport rate with increasing Li-TFSI content would appear to contradict a beneficial, or indeed significant influence of Increasing conductivity of Spiro-OMeTAD with increasing Li-TFSI content. In addition to the slowing transport rate, we also observe the charge recombination to be similarly slowed by the addition of Li-TFSI. The decrease in recombination rate has been observed before,14 and is usually attributed to the shallower DOS as well as a possible ‘‘screening’’ of the Coulomb attraction between holes in the Spiro-OMeTAD and electrons in the TiO2 by the high ion content.70,71 For a solar cell to collect charge efficiently, the absolute rates for charge collection or recombination are unimportant, but the relative magnitude of each, and specifically much slower recombination than transport is important. We show the charge collection efficiency, estimated by the ratio of the transport rate over the sum of transport and recombination rates, in Fig. 5d. For most concentrations of Li-TFSI the efficiency is 98%, indicating close to perfect compensation of slowing down recombination to slowing down transport. At very low Li-TFSI content, we observe a very rapid decrease in charge collection efficiency with charge density which is mainly due to a significant decrease in the electron lifetime at short-circuit conditions. This suggests that the electron– hole attraction at higher charge density cannot be effectively screened by the low ion content at the TiO2 interface. This, together with the known influence of lithium ions on electron injection efficiency72,73 from dye to TiO2, clearly explains the extremely large jump in measured short-circuit currents upon addition of a small quantity of Li-TFSI. We note moreover that the photocurrent generated decreases upon addition of more than 10 mol% Li-TFSI, although the charge collection efficiency remains high. This suggests that addition of excess Li-TFSI, and resulting high levels of oxidized Spiro-OMeTAD may actually hinder regeneration of the hole in the dye after electron injection, though further work is required to fully understand this photocurrent reduction. Hence, as far as charge generation and charge collection at short-circuit are concerned, it appears that a relatively low concentration of Li-TFSI would be preferable. However, from the device results we observe a continuous increase in fill factor with increasing Li-TFSI content, which results in the optimum concentration for efficiency being much higher that that optimised for photocurrent alone. The fill factor of ss-DSSCs is determined by many factors, one of the most important of which can be the series resistance due to poor charge transport properties of the hole transporter. We note that although the transport at short-circuit is limited by slow electron diffusion through the TiO2, under forward bias conditions near open-circuit the charge density in the film increases considerably, significantly increasing the effective diffusion coefficient and conductivity in the TiO2 due to the filling of sub-band-gap trap states. Transport is then likely to be limited by hole migration This journal is c the Owner Societies 2013 Fig. 6 Series resistances as measured by fitting JV curves to an ideal diode model (black), calculated series resistances from geometrical estimations using the conductivity of Spiro-OMeTAD as the only input parameter (grey), and the associated power losses estimated by a simulation of the JV curves without any series resistance (open circles, right axis), see ESI† for details. through the Spiro-OMeTAD which does not have such a strong charge density dependence upon its hole-mobility.38 To specifically quantify this effect of lithium doping on the Spiro-OMeTAD on the current–voltage characteristics, we have fitted the device current– voltage curves using a one-diode model (see ESI† for additional details) to extract the series resistance and the power losses at the maximum power point as function of the Li-TFSI content (Fig. 6).74 We note that the series resistance follows a very similar trend to the conductivity of Spiro-OMeTAD, confirming that the hole transport is limiting the charge conduction in the devices at higher applied bias. This fits well with deduction by Fabregat-Santiago et al. from impedance spectroscopy on similar devices.38 Indeed, when we estimate an ‘‘effective’’ charge transport series resistance due to Spiro-OMeTAD as a function of lithium content from the conductivity values in Fig. 1 (precise details shown in the ESI†), the estimated values for series resistance are in extremely good agreement with those estimated from the ideal diode fits to the JV curves. We estimate the power loss due to series resistance, by simulating a JV curve will all parameters identical to the one-diode fits, except the series resistance is reduced to zero. These losses are shown in Fig. 6, where we observe highly significant loses for all Li-TFSI contents below 16 mol%, and still 20% losses at 16 mol% Li-TFSI. It is evident that an effective conductivity above 105 S cm1 is necessary to produce devices that are not significantly limited by a series resistance, especially once the shortcircuit photocurrents are pushed towards 20 mA cm2.75 However, the detrimental effect of high Li-TFSI concentration on the device charge transport, DOS, and charge generation results in an overall compromise for maximum power conversion efficiency. Using a higher mobility hole-transporter to enable conductivities above 104 S cm1 at lower Li-TFSI contents, would dramatically improve the solar cell efficiency with all else being equal. To be more quantitative about the hole-transporter requirements, we can predict the conductivity values possible for holetransporters with different hole mobilities at varying doping levels, Phys. Chem. Chem. Phys., 2013, 15, 2572--2579 2577 View Article Online Published on 12 December 2012. Downloaded by University of Oxford on 08/06/2014 16:40:01. PCCP Paper assuming 100% doping efficiency (see ESI† for details). When a hole-transporter such as Spiro-OMeTAD with hole mobility on the order of 2 104 cm2 V1 s1 (an upper-end value based on timeof-flight measurements)76 is used, a conductivity around 104 S cm1 can be attained with just 1–4 mol% doping. The maximum conductivity attainable for this material, taking 10–20 mol% doping to be a practical limit and assuming a constant mobility, is then 1–2 103 S cm1. We note that the effective conductivities expected at these doping levels in a porous TiO2 matrix is expected to be lower due to an increase in physical and energetic disorder (roughly a factor of three based on the volume fraction occupied at 80% pore filling), and that the time-of-flight mobility should be an upper estimate, so that this really is an upper limit estimate. We can then conclude that a hole transporter with hole mobility on the order of 103 cm2 V1 s1 would allow for high conductivity values on the order of 103 S cm1 upon doping below 5 mol%, which should allow for a more optimized system. Conclusions In conclusion, we have shown that the lithium salts commonly added to OSs can be involved in a redox reaction, which leaves an unpaired electron in the organic matrix. The mechanism we described demonstrates that the Li+ is an efficient and stable p-dopant in the presence of oxygen. This lithium doping mechanism clarifies the dramatically increase in conductivity observed in many lithium salt–hole transporter systems, previously described in literature.22,24–28 After we established the lithium doping, we discussed the complex role of Li-TFSI in Spiro-OMeTAD based ss-DSSCs, specifically looking at its influence on conductivity of the hole-transporter. We have demonstrated that Li+ is essential in achieving low series resistances in the solar cells, while it also facilitates photocurrent generation. The first is accomplished by oxidation of the Spiro-OMeTAD, during which lithium ions are consumed, while the latter is dependent on the availability of these ions to intercalate into the TiO2 and influence the local polarity at the dye-sensitized heterojunction. It seems then that the current system employed in making ss-DSSCs is far from optimum, and that the additive is required for two opposing processes, one which consumes it and another which is dependent on its presence at the TiO2 interface. The lithium doping we have deduced could also explain some of the atmospheric dependencies different research groups have noted on device performance;39 the atmosphere is simply reacting with organic components of the device, which can only be expected to give variable results. 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