Charge carrier transport and contact selectivity limit the operation of PTB7-based organic solar cells of varying active layer thickness

In this work we study the different electrical loss pathways occurring during the operation of bulk heterojunction solar cells by using a variety of electrical and optical characterization techniques beyond the current density–voltage curve (J–V): Impedance Spectroscopy (IS), Charge Extraction (CE) and Transient Photovoltage (TPV). Two sets of devices are analyzed: the first is based on the donor polymer P3HT, known to provide efficient cells using thick active layers (i.e. 270 nm), and the recently developed PTB7 which offers maximum efficiencies for devices with thinner layers (i.e. 100 nm). Devices fabricated with P3HT:PC60BM are not limited by transport of carriers and large active layer thickness may be used. Importantly, increasing the active layer thickness does not modify the contact selectivity. This is supported by analysis of the diode curve measured in the dark (similar leakage currents) and by capacitance–voltage measurements (similar fullerene content covering the cathode). Under these conditions the current density curve under illumination is mainly defined by the recombination processes taking place in the bulk of the active layer. In contrast, transport of carriers and contact selectivity are both limiting factors for the PTB7:PC60BM system. In this case, best efficiencies are obtained with a low active layer thickness and a high fullerene ratio. Reduced active layer thickness minimizes undesired electrical resistances related to carrier transport through the bulk of the active layer. High fullerene content enhances the amount of fullerene molecules at the cathode leading to decreased leakage currents. Then, the overall device efficiency will be a combination of the recombination kinetics in the bulk of the active layer, undesired resistance to transport of carriers and leakage current present due to low selectivity of the contact. The use of additives has also been explored which enhances charge generation and extraction. Overall, this work provides a comprehensive guide on how to interpret results obtained from some of the most widely used optoelectronic techniques employed to analyse operating devices

High Photo-Current in Solution Processed Organic Solar Cells Based on Porphyrin Core A-π-D-π-A as Electron Donor Material

Two new conjugated acceptor-donor-acceptor (A-p-D-p-A) molecules with a porphyrin core linked by ethynylene bridges to two thiophene (1a) or thienylenevinylenethiophene (1b) units and both capped by N-ethylrhodanine have been synthesized. These compounds were used as the main electron donor moieties for bulk heterojunction small molecule organic solar cells (BHJ-SMOSC). The optimized devices, with PC71BM as the main electron acceptor molecule, show remarkable short circuit currents, up to 13.2 mA/cm2, an open circuit voltage of around 0.85 V, and power conversion efficiencies up to 4.3% under 100 W/cm2. The External Quantum Efficiency (EQE), Atomic Force Microscopy (AFM), hole mobility, Photo-Induced Charge Extraction (PICE) and Photo-Induced Transient Photo-Voltage (PIT-PV) were analyzed in devices based on 1a and 1b in order to account for differences in the final performance of the two molecules. The PIT-PV decays showed slower recombination kinetics for devices fabricated with 1b. Moreover, the EQE was greater for 1b and this is ascribed to the better nanomorphology, which allows better charge collection before carrier recombination takes place

High Photocurrent in Oligo-thienylenevinylene Based Small Molecule Solar Cells Having 4.9% Solar-to-Electrical Energy Conversion

A set of five novel oligo-thienylenevinylene organic molecules have been synthesized and characterized for use as electron donor moieties in bulk-heterojunction solution-processed organic solar cells combined with PC71BM as an electron acceptor. The results show a broad range of solar-to-electrical conversion efficiencies, with values up to 4.9% achieved with a photocurrent value as high as 12 mA cm&minus;2&nbsp;under standard measurement conditions. Moreover, another aim of this study was to determine the main limiting processes that control the final performance parameters of these devices. Photo-induced charge transfer measurements, such as charge extraction (CE), Transient Photo-Voltage (TPV) and mobility measurements, were carried out in order to determine the main loss mechanisms and to correlate them with the electron donor molecular design. &nbsp;</div

Energy alignment and recombination in perovskite solar cells: weighted influence on the open circuit voltage

In this work, we assess the possible reasons for the differences observed in open circuit voltage (VOC) in mixed cation perovskite solar cells when comparing four different hole transport materials (HTMs), namely TAE‐1, TAE‐3, TAE‐4 and spiro‐OMeTAD. All these HTMs present close chemical and physical properties. Additionally to the evaluation of the HTM influence on recombination, we find that, upon deposition of the organic HTM on top of the perovskite, there is an important change in the energy levels position, and the impact on the device VOC is discussed. We consider that this experimental observation could be general for other organic HTMs and would justify the difficulties for finding molecules and materials that could improve the efficiency of perovskite solar cells overcoming the solar‐to‐energy conversion efficiency of solar cells made using spiro‐OMeTAD as holes selective contact.EP and AVF thank MINECO (projects CTQ2013-47183 and CTQ2017- 89814-P). NM thanks the European Research Council (ERC-320441- Chirallcarbon), the CAM (Y2018/NMT-4783QUIMTRONIC-CM) and the Spanish Ministry of Economy and Competitiveness MINECO (projects CTQ2014-52045-R, CTQ2015-71154-P, CTQ2016-80042-R, CTQ2017-83531-R and CTQ2016-81911-REDT). NM also thanks the Alexander von Humboldt Foundation and the Unidad de Excelencia Severo Ochoa SEV-2016-0686 is acknowledged. EP and AV are also grateful to ICIQ and ICREA for economical support. EB and CO thank the Generalitat de Catalunya 2017 SGR668 and 2017 SGR669, the MINEICO projects MAT2016-77852-C2-1-R (AEI/FEDER, UE), MAT2015-68994-REDC and the ‘‘Severo Ochoa’’ Program for Centers of Excellence in R&D (SEV-2015-0496). IG thanks Jesús Jiménez López for the fruitful discussions.Peer reviewe

Energy Alignment and Recombination in Perovskite Solar Cells: Weighted Influence on the Open Circuit Voltage

In this work, we assess the possible reasons for the differences observed in open circuit voltage (VOC) in mixed cation perovskite solar cells when comparing four different hole transport materials (HTMs), namely TAE-1, TAE-3, TAE-4 and spiro-OMeTAD. All these HTMs present close chemical and physical properties, however, once they are finally deposited onto the perovskite layer, the HTMs provide different performance characteristics. Additional to the evaluation of the HTM influence on recombination, we find that, upon deposition of the organic HTM on top of the perovskite, there is an important change in the energy level position, and the impact on the device VOC is discussed. We consider that this experimental observation could be general for other organic HTMs and would justify the difficulties in finding molecules and materials that could improve the efficiency of perovskite solar cells overcoming the solar-to-energy conversion efficiency of solar cells made using spiro-OMeTAD as a hole selective contact