Diphenylmethanofullerenes: New and Efficient Acceptors in Bulk‐Heterojunction Solar Cells

Abstract

A novel fullerene derivative, 1,1‐bis(4,4′‐dodecyloxyphenyl)‐(5,6) C61, diphenylmethanofullerene (DPM‐12), has been investigated as a possible electron acceptor in photovoltaic devices, in combination with two different conjugated polymers poly[2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐para‐phenylene vinylene] (OC1C10‐PPV) and poly[3‐hexyl thiophene‐2,5‐diyl] (P3HT). High open‐circuit voltages, VOC = 0.92 and 0.65 V, have been measured for OC1C10‐PPV:DPM‐12‐ and P3HT:DPM‐12‐based devices, respectively. In both cases, VOC is 100 mV above the values measured on devices using another routinely used fullerene acceptor, [6,6]‐phenyl‐C61 butyric acid methyl ester (PCBM). This is somewhat unexpected when taking into account the identical redox potentials of both acceptor materials at room temperature. The temperature‐dependent VOC reveals, however, the same effective bandgap (HOMOPolymer–LUMOFullerene; HOMO = highest occupied molecular orbital, LUMO = lowest unoccupied molecular orbital) of 1.15 and 0.9 eV for OC1C10‐PPV and P3HT, respectively, independent of the acceptor used. The higher VOC at room temperature is explained by different ideality factors in the dark‐diode characteristics. Under white‐light illumination (80 mW cm–2), photocurrent densities of 1.3 and 4.7 mA cm–2 have been obtained in the OC1C10‐PPV:DPM‐12‐ and P3HT:DPM‐12‐based devices, respectively. Temperature‐dependent current density versus voltage characteristics reveal a thermally activated (shallow trap recombination limited) photocurrent in the case of OC1C10‐PPV:DPM‐12, and a nearly temperature‐independent current density in P3HT:DPM‐12. The latter clearly indicates that charge carriers traverse the active layer without significant recombination, which is due to the higher hole‐mobility–lifetime product in P3HT. At the same time, the field‐effect electron mobility in pure DPM‐12 has been found to be μe = 2 × 10–4 cm2 V–1 s–1, that is, forty‐times lower than the one measured in PCBM (μe = 8 × 10–3 cm2 V–1 s–1).

The Origin of the High Voltage in DPM12/P3HT Organic Solar Cells

“…[ 8 , 9 ] Organic solar cells based on a mono-substituted fullerene 1,1-bis(4,4 ′ -dodecyloxyphenyl-(5,6)C 61 , diphenylmethanofullerene (DPM12) have been previously reported with effi ciencies reaching 2.3% (80 mW cm − 2 , 1.5 AM G). [ 13 ] Our DPM12 -based organic solar cells showed a V oc that was 100 mV higher under 100 mW cm − 2 sun-simulated light intensity and an overall better device performance than PCBM/P3HT devices when illuminated with light of 10-50 mW cm − 2 intensity, which is in good agreement with the above mentioned results. For this paper, we used the TPV technique with the aim of comparing our DPM12/P3HT devices with standard PCBM/P3HT solar cells and explaining the differences in the observed V oc under normal measurement conditions.…”

Influence of Phase Segregation on Recombination Dynamics in Organic Bulk‐Heterojunction Solar Cells

“…Similar to PPV:PC 61 BM blend systems, [ 7 ] it was suggested for pBTTT:PC 71 BM devices-which are chemically related to the blend system studied in this work-that the addition of more than 50 wt.% fullerene would lead to phase segregation, with PC 71 BM domains growing in proportion to the amount of fullerene added. [ 5 ] Thus, although residual PC 61 BM molecules in the 1:4 blends do not contribute to a further increase in the lamellar distance, the changed phase segregation does affect the transport properties and recombination dynamics of BHJ solar cells, which has been suggested previously, [12][13][14] and will be discussed below.…”

Origin of the dark-current ideality factor in polymer:fullerene bulk heterojunction solar cells

“…11,16 In our recent work, 14 we showed a temperature independent ideality factor for single-carrier devices. If the dark current of the blend has an ideality factor equal to that of PCBM, the blend ideality factor should also be temperature independent.…”