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Current voltage curves acquired under simulated solar illumination (100 mW/cm2) for the different top contacts are shown in Figure. 3.18 Calculated characteristics are summarized in Table 3.3. The FF is defined as

FF=IMPP∙VMPP/ ISC∙VOC

where ISC denotes the short cut current, VOC the open circuit voltage and IMPP and VMPP the current and voltage at the maximum power point, respectively. RSh and RS are obtained following the generalized Shockley diode model and calculated from linear fits to the I-V curves at small reverse bias and moderate forward bias, respectively.

Both devices without PEDOT:PSS exhibit reasonable photocurrents but low FF around 35 %, low VOC of about 400 mV and overall PCE below 1 %. In contrast, the devices utilizing PEDOT:PSS provide PCE values of 1.96 % and 2.44 % and FF above 50 % and 55 % for evaporated and sputtered Ag, respectively. Additionally, the VOC is enhanced to almost 0.6 V for both metal contacts due to introduction of a PEDOT:PSS interlayer (Table 3.3).

Figure 3.18 I-V characteristics under illumination with simulated AM 1.5G solar light (100 mW/cm2_{). Data is} shown for devices with PEDOT:PSS (circles: thermally evaporated Ag, stars: sputtered Ag) and without PEDOT:PSS (squares: thermally evaporated Ag, triangles: sputtered Ag). Devices with PEDOT:PSS exhibit higher VOC and FF.

The significantly better performance with PEDOT:PSS is attributed to two different effects. Ideally, the donor‘s HOMO and the acceptor‘s LUMO match the work functions of the hole and electron collecting electrodes, respectively. Ohmic contacts between the active materials and the external electrodes allow high photocurrents due to good charge extraction and high VOC [171]. PEDOT:PSS exhibits a work function of -5.0 eV matching the HOMO of P3HT better than the bare silver contact which has a work function between -4.5 eV and -4.7 eV [172] (compare Figure 3.17). Besides, PEDOT:PSS is known to accept holes from various donor materials even if the respective HOMO level doesn‘t fit its work function [173]. The improved energy level matching is also reflected in relatively lower Rs values for the devices with PEDOT:PSS.

The second effect caused by the PEDOT:PSS might be the protection of the P3HT:PCBM layer during the top contact deposition. Sputtered Ag-atoms are assumed to have higher energy when hitting the organic film (deposition rate about 1.0 Å /s). Additionally, when sputtering Ag clusters of different size are deposited next to single atoms [174]. Therefore, silver might

3.3 Spray-deposited PEDOT: PSS for air stable inverted organic solar cells

penetrate into the active layer especially in case of the sputtering [175]. This is also reflected in low RSh indicating leakage currents that are limiting the device performance and reducing the FF. RSh is only 231 Ω∙cm

2

for the evaporated Ag and even lower (154 Ω∙cm2) for the sputtered Ag, probably due to the deeper penetration into the organic layer during the sputtering. Leakage currents are known to also affect the VOC if RSh is reduced to a certain point [176] which also might partly explain the observed low VOC values for the devices without PEDOT:PSS. However, we attribute the lower VOC mainly to a non-ideal work function of the fresh prepared Ag top contact as discussed later.

PCE [%] VOC [V] ISC [mA/cm2] FF [%] RSh [Ω∙cm2 ] RS [Ω∙cm2 ] no PEDOT, Ag evap 0.92 0.41 6.29 34.1 231 26.9 PEDOT, Ag evap 1.96 0.58 6.32 51.7 638 17.6 no PEDOT, Ag sput 0.96 0.42 6.34 34.7 154 14.7 PEDOT, Ag sput 2.44 0.58 7.11 56.9 1019 2.62

Table 3.3 Calculated characteristics from current-voltage data acquired under simulated solar illumination.

Values are given for solar cells with and without PEDOT:PSS and with thermally evaporated and sputtered Ag top contact.

The higher photocurrent observed for the sputtered compared to the evaporated top contacts with PEDOT:PSS is attributed to a better contact between the silver and the PEDOT:PSS. Very likely sputtered Ag penetrates deeper into the PEDOT:PSS than slowly evaporated Ag allowing for an unhampered hole injection into the silver electrode. This is also reflected in lower Rs values for the sputtered top contacts for both solar cell types, with and without PEDOT:PSS. The reduced Rs is also the main reason for the high FF of the device with PEDOT:PSS and sputtered Ag.

Figure 3.19 Dark I-V curves. Devices with PEDOT:PSS (circles: thermally evaporated Ag, stars: sputtered Ag) show higher currents in forward direction a shape indicating no space charge limits.Cells without PEDOT:PSS (squares: thermally evaporated Ag, triangles: sputtered Ag) seem to be space charge limited and allow only significantly lower forward currents.

Further hints for better hole collection with PEDOT:PSS are provided by the dark I-V curves in Figure 3.19 The devices with PEDOT:PSS allow a much higher current in forward direction and an enhanced rectifying behavior. Moreover, the differences in shape of the I-V curves with and without PEDOT:PSS at moderate forward bias indicate space charge limits in the devices without PEDOT:PSS [177, 178]. Since the electron collection in the TiO2 should be similar with and without PEDOT:PSS and P3HT:PCBM solar cells are known to be space charge limited mainly due to slow hole transport [32] we attribute this to space charge forming at the organic-silver interface if no PEDOT:PSS is used. In contrast, if the PEDOT:PSS interlayer ensures an ohmic contact between the P3HT and the top electrode holes can exit the device unhamperedly.

The observed variations of the reverse current in the dark I-V curves also correspond to the deeper penetration of the sputter Ag into organic layers. The device without PEDOT:PSS and the sputtered Ag exhibits the highest current at reverse bias, i.e. the highest leakage whereas the reverse current is significantly reduced if PEDOT:PSS protects the organic layer during the

3.3 Spray-deposited PEDOT: PSS for air stable inverted organic solar cells

sputtering.

The overall relatively low performance of about 2.5 % compared to record values of 5-6 % is attributed to two facts. First, our fabrication does not include any processing steps in an inert nitrogen atmosphere. Both spin coating and annealing are performed in ambient air. Secondly, the substrates used are suffering at the elevated temperatures used for spray pyrolysis of the TiO2 layers. This increases series resistances caused by the ITO which loses conductivity at the high temperatures during the TiO2 spray pyrolysis. Additionally, our active area of 0.125 cm2 is relatively large.