Investigations of efficiency improvements in poly(3-hexylthiophene) based organic solar cells using calcium cathodes

Bridging donor–acceptor energy offset using organic dopants as energy ladders to improve open-circuit voltages in bulk-heterojunction solar cells

“…Comparison of the results via the UPS spectra of the doped blend (as indicated in the bottom of Fig. 5(a)) with corresponding data from the pristine blend, which are naturally sensitive to the composed heterojunction within the top 20-Å surface, provides direct evidence of the top TCTArich surface for the doped blend instead of the top P3HT-rich surface addressed in previous work [4]. As established in the earlier study, thermally induced out-diffusion of PCBM molecules to the top surface improves the transport across the heterojunction and shows a HOMO shift of 0.2 eV in the UPS spectrum under thermal annealing [29].…”

“…Increasing open-circuit voltage (V oc ) would be one of the key steps to improve the performance of organic photovoltaic devices. Among many elements that control V oc , the energy band alignments of the donor-acceptor in the BHJ devices have been reported as the major factor that limit the value of V oc [3][4][5]. Since V oc in donor-acceptor systems correlates to the energetic difference between the highest occupied molecular orbital (HOMO) of the electron donor and lowest unoccupied molecular orbital (LUMO) of the electron acceptor [6][7][8], one of the route toward a larger V oc is to incorporate low-band gap semiconducting polymers with deeper HOMOs as donors [9,10] and electron-accepting fullerene derivatives with shallower LUMOs as acceptors [11,12].…”

Investigation on polymer solar cells by using calcium as an Electron Transportation Layer

“…Wu [17] found the improvement mainly attributed to the shift of the Highest Occupied Molecular Orbit (HOMO). The Fermi Energy Level (E F ) moved towards the Lowest Unoccupied Molecular Orbit (LUMO) of P3HT.…”

“…Meanwhile, the short circuit current (I sc ) remains steady because of a low resistance contact between active layer and cathode interface [17][18][19][20] after Ca was initially inserted (Group 1-3). However, I sc decreases after 30 nm Ca was inserted.…”

“…Moreover, leakage current [12,13] in OPVs still needs to be clarified. In this paper, Calcium (Ca) was used as ETL because of its lower electro-negativity [14], higher conductivity [15], and better matching with aluminum (Al) [16].…”