SUPPLEMENTARY INFORMATION

Transcription

1 SUPPLEMENTARY INFORMATION On the origin of the open-circuit voltage of polymer:fullerene solar cells Koen Vandewal, Kristofer Tvingstedt, Abay Gadisa, Olle Inganäs and ean V. Manca The additional information consists of 3 parts. In part 1 EQE PV (E) and EQE EL (E) spectra and their relation are shown for all investigated devices, with particular focus on the impact of various preparation conditions. In Part 2, a comparison is made between the 0 values obtained by analysis of the EQE PV (E) and EQE EL (E) spectra, as explained in the main text and 0 as obtained directly from the IV curves. In part 3, additional information relating the equation (4) used in the main text to calculate V oc is provided. 1. Determination of 0 by analysis of EQE PV (E) and EQE EL (E) for devices using various preparation conditions For one particular polymer:fullerene combination, the open-circuit voltage (V oc ) can depend on the preparation conditions. Therefore, this supplementary information contains photovoltaic external quantum efficiency (EQE PV ), electroluminescene external quantum efficiency (EQE EL ) spectra and the 0 values for additional devices, using various preparation conditions. The 0 values were obtained from the EQE PV and EQE EL spectra, as described in the main text. For the MDMO-PPV and APFO3 based devices, 3 different polymer:fullerene stoichiometries were investigated. APFO3 was blended with PC 61 BM and PC 71 BM. For the devices using PCPDTBT as donor polymer, the effect of the additive 1,8- octanedithiol in the blend solution was studied. For P3HT:PC 61 BM spectra were obtained for the annealed and unannealed devices. For the MDMO-PPV:PC 61 BM, APFO3:PC 61 BM and APFO3:PC 71 BM devices, polymer:fullerene ratios of 20:80, 50:50 and 80:20 were used, shown in respectively additional figure 1, additional figure 2 and additional figure 3. Upon increasing the fullerene content, a redshift of the CT band in both the EQE PV and EQE EL spectrum can be observed. A Redshift of the CT band upon increasing the fullerene content was observed previously in FTPS [1], photoluminescence [2] and electroluminescence [3] nature materials 1

2 supplementary information studies. This redshift is the main cause of the increasing 0 with increasing fullerene content. At this point it is difficult to see a trend in the absolute EQE EL values as function of varying fullerene content. EQE EL will depend on many parameters, including the used contacts and the quality of the contact of the used metal with the active layer. The addition of additives to the PCPDTBT:fullerene solution has been shown to be beneficial for the overall power conversion efficiency of the photovoltaic device. This is due to an increase in fillfactor and short circuit current. The V oc however, drops slightly [4]. Also in this case, the drop in V oc and increasing 0 upon adding 1,8-octanedithiol is caused by a redshift of the CT band (additional figure 4). Annealing of P3HT:PC 61 BM devices results in a higher efficiency device, but again, a lower V oc, as in the unannealed case. Again the origin is the redshift of the CT band upon annealing, due to the annealing induced order (additional figure 5) [5]. 2. Relating 0 obtained by analysis of EQE PV (E) and EQE EL (E) to 0 obtained by analysis of the IV curves In equation (2) of the main paper a diode equation was used, having the form qv = 0 exp 1 kt inj (AI1) As emphasized in the main text, for polymer:fullerene devices, it should be taken into account that 0 is not constant and depends on the number of charges present in the device. Therefore it is important in our analysis that EQE EL is measured as close as possible to 1 sun conditions. Due to the limited sensitivity of the detector, we could measure EQE EL for injection currents corresponding to 1-10 times sc, depending on the material system. For dark IV curves of polymer:fullerene solar cells, the following function, including an ideality factor n is often used to fit the diodelike part of the curve: 2 nature MATERIALS

3 supplementary information qv = 0, n exp 1 nkt inj (AI2) In this case 0,n is a constant. It is the intercept with the 0 V line of an exponential fit of the injected current inj (V). values for n and 0,n for the 5 example material systems are determined from the dark IV curves shown in additional figure 6 and are listed in additional table 1. A relation between 0, as defined in the main paper, and 0,n can be deduced by equating (AI1) and (AI2): qv 0 exp 1 = 0, n exp kt qv nkt 1 At voltages V exceeding several times nkt, we can neglect the -1 term on both sides of the equation 0 0 qv exp = 0, n exp kt (1 n) qv = 0, n exp nkt qv nkt Or, in function of inj : 0 = n 1 n 0, n inj At ideality factors n>1 it is necessary to evaluate 0 under the right conditions. As the papers deals with 1 sun conditions, we must evaluate this expression at inj = ph. We get the following expression for 0 in function of IV curve parameters. n = 0 0, n n sc sc (AI3) It is now possible to compare 0 determined by the approach presented in the main paper with 0 evaluated using equation (AI3). Their comparison is shown in additional figure 7. nature materials 3

4 supplementary information The agreement between 0 obtained via EQE PV and EQE EL and the 0 obtained from the IV curves is reasonable, and a trend is clearly visible. However, 0 obtained via EQE PV and EQE EL seems to be a slight overestimation of the 0 obtained via IV curves. This is because 0 was evaluated from the EQE PV (E) spectra obtained at short-circuit and not at injection conditions at a voltage comparable to V oc. Due to the field dependent photocurrent in some material systems the EQE PV under injection conditions can be lower than the EQE PV at short circuit, resulting in an overestimation of 0 via the method presented in the main paper. However this overestimation of 0 does not affect our calculation of V oc, as explained in the next section of this additional information. 3. Relating 0 to V oc At V oc the produced photocurrent ph balances with the injected current inj, and we get the following formula for V oc from equation (AI1) or equation (2) of the main paper. V oc kt ph = ln + 1 * q 0 Hereby are ph and 0 * evaluated by integrating the product of EQE PV (E) with respectively the AM1.5 spectrum and φ BB (E). If there is a voltage dependence of the photocurrent ph, EQE PV (E) will also depend on voltage and for the evaluation of ph and 0 *, EQE PV (E) should be measured under injection conditions, at voltages comparable to V oc. Assuming that the spectral shape of EQE PV (E) does not depend on the injection conditions than the ph / 0 * ratio depends only on the spectral shape of the EQE PV (E) spectrum and we can replace ph / 0 * with sc / 0. Hereby are sc and 0 evaluated by integrating the product of the EQE PV (E) spectrum measured at short circuit with respectively the AM1.5 spectrum and φ BB (E), as described in the main paper. We obtain thus equation (5) of the main paper V oc kt sc = ln + 1 q 0 Hereby should 0 be evaluated as in the main paper, with EQE PV (E) measured at short-circuit. 4 nature MATERIALS

5 supplementary information References _[1] Vandewal K. et al. The relation between open-circuit voltage and the onset of photocurrent generation by charge-transfer absorption in polymer: fullerene bulk heterojunction solar cells. Adv. Funct. Mater. 18, (2008). _[2] Veldman D. et al. Compositional and electric field dependence of the dissociation of charge transfer excitons in alternating polyfluorene copolymer/fullerene blends.. Am. Chem. Soc. 130, (2008). _[3] Tvingstedt K. et al. Electroluminescence from charge transfer states in polymer solar cells.. Am. Chem. Soc. article ASAP (online August 4) (2009). _[4] Peet. et al. Efficiency enhancement in low-bandgap polymer solar cells by processing with alkane dithiols. Nat. Mater. 6, (2007). _[5] Campoy-Quiles M. et al. Morphology evolution via self-organization and lateral and vertical diffusion in polymer: fullerene solar cell blends. Nat. Mater. 7, (2008). nature materials 5

6 supplementary information Figures Additional figure 1: EQE PV (E), EQE EL (E) and 0 for MDMO-PPV:PC 61 BM in 3 different stoichiometries. The used polymer:fullerene ratios are 20:80 (blue), 50:50 (green) and 80:20 (red). Panel (a) shows the EQE PV spectra. EQE EL and the product of EQE PV with the blackbody spectrum at room temperature are shown in panel (b). Panel (c) shows 0, calculated using formula (3) of the main text. 6 nature MATERIALS

7 supplementary information Additional figure 2: EQE PV (E), EQE EL (E) and 0 for APFO3:PC 61 BM in 3 different stoichiometries. The used polymer:fullerene ratios are 20:80 (blue), 50:50 (green) and 80:20 (red). Panel (a) shows the EQE PV spectra. EQE EL and the product of EQE PV with the blackbody spectrum at room temperature are shown in panel (b). Panel (c) shows 0, calculated using formula (3) of the main text. nature materials 7

8 supplementary information Additional figure 3: EQE PV (E), EQE EL (E) and 0 for APFO3:PC 71 BM in 3 different stoichiometries. The used polymer:fullerene ratios are 20:80 (blue), 50:50 (green) and 80:20 (red). Panel (a) shows the EQE PV spectra. EQE EL and the product of EQE PV with the blackbody spectrum at room temperature are shown in panel (b). Panel (c) shows 0, calculated using formula (3) of the main text. 8 nature MATERIALS

9 supplementary information Additional figure 4: EQE PV (E), EQE EL (E) and 0 for PCPDTBT:PC 61 BM devices, with and without the use of 1,8-octanedithiol. The spectrum in green is the as prepared device, the spectrum in red is with the use of 1,8-octanedithiol (ODT). Panel (a) shows the EQE PV spectra. EQE EL and the product of EQE PV with the blackbody spectrum at room temperature are shown in panel (b). Panel (c) shows 0, calculated using formula (3) of the main text. nature materials 9

10 supplementary information Additional figure 5: EQE PV (E), EQE EL (E) and 0 for annealed and unannealed P3HT:PC 61 BM devices. The unannealed device is shown in the green spectrum while the annealed device is shown in red. Panel (a) shows the EQE PV spectra. EQE EL and the product of EQE PV with the blackbody spectrum at room temperature are shown in panel (b). Panel (c) shows 0, calculated using formula (3) of the main text. 10 nature MATERIALS

11 supplementary information Additional figure 6: The dark injected current inj (V) versus voltage V for polymer:fullerene devices. Active layers of the devices are: P3HT:PC 61 BM (1:1) (annealed) (purple), PCPDTBT: PC 61 BM (1:2) (green), LBPP5: PC 71 BM (1:3) (red), MDMO-PPV: PC 61 BM (orange) (1:4) and APFO3: PC 61 BM (1:4) (cyan). The red lines represent exponential fits, allowing determination of 0,n and the ideality factor n. nature materials 11

12 supplementary information Additional figure 7: 0 obtained via analysis of the EQE PV (E) and EQE EL (E) spectra, compared with 0 obtained from 0,n and n. Active layers of the devices are: P3HT:PC 61 BM (1:1) (annealed) (purple), PCPDTBT: PC 61 BM (1:2) (green), LBPP5: PC 71 BM (1:3) (red), MDMO-PPV: PC 61 BM (orange) (1:4) and APFO3: PC 61 BM (1:4) (cyan). The gray line represents a one to one relation. 12 nature MATERIALS

13 supplementary information Additional table 1: Values for 0,n and n obtained by fitting of the exponential part of the dark IV curves. Material system 0,n (A.m -2 ) n P3HT:PCBM (1:1) annealed 2.5E PCPDTBT:PCBM (1:2) 4.0E LBPP5:PC70BM (1:3) 2.0E MDMO-PPV:PCBM (1:4) 5.2E APFO3:PCBM (1:4) 4.6E nature materials 13