ITO Interface Modifiers Can Improve V_OC in Polymer Solar Cells and Suppress Surface Recombination

Abstract

We use dipolar phosphonic acid self-assembled monolayers (PA SAMs) to modify the work function of the hole-extracting contact in polymer/fullerene bulk heterojunction solar cells. We observe a linear dependence of the open-circuit voltage (V OC ) of these organic photovoltaic devices on the modified indium tin oxide (ITO) work function when using a donor polymer with a deep-lying ionization energy. With specific SAMs, we can obtain V OC values exceeding those obtained with the common poly(3,4ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) hole-extraction layer. We measure charge-carrier lifetimes and densities using transient photovoltage and charge extraction in a series of devices with SAM-modified contacts. As expected, these measurements show systematically longer carrier lifetimes in devices with higher V OC values; however, the trends provide useful distinctions between different hypotheses of how transient photovoltage decays might be controlled by surface chemistry. We interpret our results as being consistent with changes in the band bending at the ITO/bulk heterojunction interface that have the net result of altering the internal electric field to help prevent electrons in fullerene domains from undergoing surface recombination at the hole-extracting electrode. SECTION: Energy Conversion and Storage; Energy and Charge Transport

Amorphous oxide alloys as interfacial layers with broadly tunable electronic structures for organic photovoltaic cells

“…Besides the trend in PCE, the variations in FF also closely track the ΔE LUMO-acceptor -E CBM-oxide energy offsets described above. This finding confirms that interfacial energy alignment has a direct consequence for device FF attributed to variation of interfacial charge transport properties which ultimately affect the OPV series and shunt resistance (10,17,60,61). From these results, the significance of IFL CBM tuning for emerging OPV systems is clearly evident.…”

“…These IFLs function to accommodate the cell built-in fields, to assist carrier extraction, and to suppress parasitic carrier recombination (2,(14)(15)(16). Indeed, studies on interfacial modification of HT electrodes verify that changing the band alignment profoundly affects OPV open circuit voltage (V oc ) and carrier lifetimes (17). Over the past decade, extensive efforts have focused on band gap engineering of OPV active layer materials, mainly by tuning the donor material highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energetics to enhance built-in potentials and maximize photon harvesting efficiency (18,19).…”

Recent Advances in Bulk Heterojunction Polymer Solar Cells

“…241 A linear dependence of V oc on modified ITO work function is observed. Devices with higher V oc showed longer carrier lifetimes, which is attributed to the suppression of surface bimolecular recombination of charge carriers at the HSL/BHJ interface.…”

Effective Approaches to Improve the Electrical Conductivity of PEDOT:PSS: A Review

“…[ 141 ] ITO, which is conventionally used as the standard transparent electrode of OLED and PSC, has problems of scare indium on earth and high mechanical brittleness. [142][143][144] As discussed in section 2, PEDOT:PSS can obtain high conductivity through secondary treatments, and the maximum value is comparable to that of ITO. Especially, the work function of PEDOT:PSS is about 5.2 eV, higher than that of ITO (4.7 eV), and it is hardly affected by treatments.…”