Skip to main content

Solution Processed Next Generation Photovoltaics

Periodic Reporting for period 4 - Sol-Pro (Solution Processed Next Generation Photovoltaics)

Reporting period: 2020-01-01 to 2020-06-30

Light is one of the most important goods on earth: The sun is the single most clean and sustainable energy source which can solve all the energy needs of our world. The energy in sunlight striking the earth for 40 min is equivalent to global energy consumption for a year. During the last years, increasing concern about global warming has led to an intense search for cost-effective alternative energy sources such as solution processed photovoltaics (PVs). During the Sol-Pro ERC project a systematic understanding of the relationship between metal oxide-based electronic materials, processing and device performance relevant to Hybrid Perovskite PVs (PVSKs), Organic PVs (OPVs) and inorganic solution processed PVs, research and development targets was performed. The ERC project developed solution processed PV components relevant to product development targets [power conversion efficiency (PCE), lifetime (LT), cost) for next generation solution processed PVs.
In the case of Inverted Hybrid Perovskite (PVSKs) best PCE of 20.45 % achieved by using a triple cation perovskite formulation, while more reliable nitrobenzene additive based methylammonium-free (CsFA) PVSK formulation provide an increased mean PCE from 15.34% to 17.09%, with a much narrower PCE standard deviation distribution (please see figure 1). The improved performance is attributed to the interaction of perovskite’s colloidal particles with nitrobenzene, as well as passivation of grain boundary defects (A. Ioakeimidis & S. A. Choulis, Materials, 2020). A range of solution processed metal oxides suitable for carrier selective contacts such as the solution combustion synthesized (SCS) un-doped and doped NiCo2O4 (I.T. Papadas, et al, Advanced Science, 2018, and A. Ioakeimidis, et al., APL Materials, 2019), dellaffosite CuGaO2 (I. Papadas, et al, Materials Today Energy, 2018) and the binary CuAlO2/Cu-O (A Savva, et al, ACS Appl. Energ. Mat, 2019) developed and their application for high performance PVSKs was demonstrated. The β-alanine surface treatment process on SCS based carrier selective contacts eliminated major PVSK degradation mechanisms, providing, J/V hysteresis-free, efficient, and thermally stable PVSKs. Based on this development (β-alanine surface treated HTLs) we have achieved as shown in figure 2 greatly improved (from 24 h for the un-treated inverted PVSCs to 1000 h for the β-alanine-surface treated Cu:NiOx HTL based inverted PVSKs) thermal device stability (F. Galatopoulos, et al., Submitted to Nanomaterials, 2020). Furthermore, the addition of the nitrobenzene within the reliable methylammonium-free (CsFA) perovskite formulation increased humidity lifetime performance (75% RH and 22 °C) by 10 times due to defect passivation and inhibition of moisture permeation (A. Ioakeimidis and S. Choulis, Materials, 2020).
In the case of Inverted OPVs, by using non-fullerene acceptors inverted OPVs with PCE of 13.85 % have been achieved but lifetime limitations have been identified. Sol-Pro trials to replace the commonly used thermally evaporated MoO3 HTL with a high-performance/stable solution processed metal-oxide HTL for inverted OPVs did not produce reliable devices. In the case of inverted OPVs the best performing solution processed, hole selective contact remains the PEDOT:PSS combined with Dynol/Zonyl additives (F. Hermerschmidt, et al, ACS Appl. Mater. and Interf., 2017). As shown in figure 3, the Sol-Pro project developed a high-performance low temperature solution processed electron selective contact consisting of 10 at% antimony doped tin oxide (ATO) and the neutral polymer polyethylenimine (PEI) for high performance inverted OPVs (E. Georgiou, et al., APL Materials 2019). Furthermore, low cost solution ITO-free OPVs were developed consisting of Ag NWs and doped metal oxide-based carrier selective contact (E. Georgiou, et al, Manuscript under preparation, 2020). As indicated within the Sol-Pro action important Sol-Pro developments on PVs were also applied to close related applications such as organic light emitting diodes (OLEDs). We have shown that metal oxide (MO) interfacial layers inserted between ITO and PEDOT:PSS can be used to improve the bottom OLEDs electrodes. Best performance was achieved using the Sol-Pro SCS developed co-doped NiCo2O4, for which the current efficiency and luminous efficacy of SY OLEDs increased, respectively, by 12% and 11% (S. M. Pozov, et al., Submitted to Organic Electronics, June-2020).
In the case of all-oxides solution processed PVs. A method to assemble networks of strongly connected metal oxide nanoparticles was performed by short ligand and a polar aprotic solvent for the colloidal dispersion of SCS-based MnFe2O4 (MFO) NPs. Figure 4 shows characterisation studies for relevant PV properties of the MnFe2O4 nanoparticles and thin films. The proposed strategy is crucial to obtain functional all oxides photovoltaic devices that are fabricated by solution processing. All solution processed inorganic MnFe2O4 based heterojunction photovoltaics were fabricated. The corresponding all-oxides solar cells reveal a high open circuit voltage of 1.17 V with a fill factor of 51.2 % but a limited short circuit current of 0.07 mA cm-2, delivering PCE of 4.2 10-2 % under 100 mW cm-2 illumination. Despite the low PCE values of the novel SCS based all inorganic PVs, the obtained Voc values are much higher compared to Voc values of all-oxide solution processed based PVs reported in the literature such as [BiFeO3 (∼0.41 V)] and [Pb(ZrTi)O3 (~0.6 V)] (I. T. Papadas, et al., Manuscript under preparation, 2020).
During the ERC project, 14 Sol-Pro papers are already published, 2 are under review and at least 2 more are under preparation for journal publication. The synthesis methods (solution combustion synthesized, surfactant hydrothermal synthesized, and colloidal solvothermal synthesized) of metal oxide electronic materials that applied within the project, provided advantages comparing to other methods that positive affect not only PCE but importantly reliability and LT performance (IT Papadas, et al, Advanced Science 2018 & I.T Papadas, et al, Nanomaterials, 2019). The Sol-Pro highlighted that many of the physical and engineering aspects that govern the behavior of next generation solution processed PVs occur at interfaces. The Sol-Pro ATO/PEI electron selective contact exhibited optimized and ideal light soaking-free PCE which is essential for reliable roll-to-roll printing OPVs. Perhaps, the largest measurable achievement of the Sol-Pro project was the 40 times increased lifetime (LT) under accelerated heat LT conditions (F. Galatopoulos, et al., Nanomaterials, 2020) and 10 times increased LT performance under accelerated humidity LT conditions (A. Ioakeimidis et al., Materials 2020). In addition to the above and relevant to LT performance impact the Sol-Pro project also introduced the following concepts for improving the stability of PVSKs a) Incorporation of diffusion blocking layers (F. Galatopoulos et al, Adv. Mat. Interf.,2018) and b) Development of intimate interfaces (I.T Papadas, et al, Nanomaterials, 2019). Indicating the importance of diffusion mechanisms, chemical stability, reduced charge traps interfacial densities relevant to LT performance of PVSKs and highlighted the importance of diffusion blocking layers and intimate interfaces for the development of long-lived next generation solution processed PVs.