Flexible perovskite solar cells and mini-modules for all-thin-film tandem photovoltaics
- Doctoral Thesis
Rights / licenseIn Copyright - Non-Commercial Use Permitted
Organic-inorganic perovskite photovoltaic (PV) technology has achieved a record efficiency approaching 24 % in less than 10 years since its first reported application in PV devices. The outstanding PV performances, high absorption coefficient, tunable band gap and low Urbach energy of this organic-inorganic material make perovskite solar cells (PSCs) ideal candidates as top cells in tandem applications. The development of tandem devices enables to improve the power conversion efficiency by reducing thermalization losses and by maximizing the absorption over the entire solar spectrum. All-thin-film perovskite/Cu(In,Ga)Se2 (CIGS) tandem solar cells in four-terminal configuration with efficiencies approaching 25 % have been demonstrated. In addition, PSCs can be efficiently prepared by low-temperature processes on flexible substrates, with reported efficiencies as high as 19 %. As well, CIGS solar cells can be developed on flexible substrates with high efficiencies (> 20 %). The development of flexible PV devices opens the way to high throughput roll-to-roll manufacturing with low embodied energy, and to new applications such as building- or transport-integrated products, as well as internet-of-thing based devices. This thesis focuses on the development of efficient near-infrared (NIR) transparent PSCs grown via low-temperature processing onto a flexible foil, which is commonly used to encapsulate flexible CIGS modules. These developments aim to lay the foundations to future manufacturing of flexible perovskite/CIGS tandem devices by direct integration of the two technologies via high-throughput roll-to-roll processing, considering that the employed flexible foil would avoid any additional costs for the top cell substrate. In addition, the upscaling potential of the developed flexible PSC structure is proved onto larger areas (from 0.15 cm2 to > 10 cm2) and highly accurate scribing methods are employed to realize monolithically-integrated flexible perovskite mini-modules. At first, a low-temperature PSC structure is developed based on an alternative highly NIR-transparent transparent conducting oxide (TCO), Al-doped ZnO (AZO). Thermally evaporated C60 is developed as low-temperature deposited electron transport layer (ETL) to efficiently extract electrons and mitigate hysteresis phenomenon in the flexible PSCs. Efficiencies above 13 % and 11 % are demonstrated on 0.15 and > 1 cm2 active areas, respectively. For tandem application, the gold rear electrode is substituted with a transparent one (appropriate combination of MoOx buffer layer and highly NIR-transparent In2O3:H TCO) to allow efficient transmission of low energy photons, resulting in a 12.2 % NIR-transparent flexible PSC with an average transmittance of 78 %, between 800-1000 nm. As a first proof-of-concept, a flexible perovskite/CIGS tandem cell with an efficiency > 18 % (higher than both single-junction devices) is demonstrated in four-terminal configuration. Through ultraviolet photoelectron spectroscopy (UPS) measurements, the interface between AZO and C60 is identified as a bottleneck for the performances of the developed flexible PSCs, due to unfavorable upward band bending at the TCO/ETL interface that hinders efficient charge extraction. The power conversion efficiency is improved by appropriate interlayer application. Solution processed polyethylenimine ethoxylated (PEIE) and vacuum deposited LiF interfacial modifications enables an absolute efficiency improvement of 2 %. A modulation of the electrostatic potential is verified once interlayers are applied, resulting in a favorable downward band bending at the AZO/C60 interface that results in enhanced electrons extraction. In addition, focusing on the use of vacuum deposited LiF interlayer, better suited for large area processing, a flexible perovskite mini-module with an efficiency of 10.5 % onto an aperture area > 10 cm2 is developed. Laser scribing method is successfully employed to realize monolithically interconnected flexible mini-module with low dead area losses. This precise and accurate approach yields a geometric fill factor (GFF) of 94 %. However, the low-temperature deposited PSC structure developed displays limited Jsc values with respect to state-of-the-art devices. A method is then presented to tackle this issue via a modified vacuum-solution (multi-stage) deposition approach, tailoring PbI2 growth to facilitate organic cations intercalation and to effectively increase perovskite thickness. Through this multi-stage deposition, an improvement in efficiencies for flexible PSCs is shown (from 14.2 % to 15.8 %) thanks to enhanced absorption in the perovskite layer. In addition, in these devices an amorphous TCO, InZnO (IZO), is used, further proving superior bending stability with respect to the initially developed crystalline AZO and to other commonly used TCOs for flexible PSCs. Flexible devices retained 90 % and 80 % of the initial efficiency after 1000 bending cycles at 6 and 4 mm bending radii, respectively. Via the multi-stage process, a flexible NIR-transparent PSC with an efficiency of 14.0 % is presented. By combining the flexible perovskite top cell with a flexible CIGS bottom cell, a flexible perovskite/CIGS tandem, with an efficiency of 19.6 % measured in four-terminal configuration is eventually demonstrated. In order to identify the main steps to improve the efficiency of the developed flexible PSCs, the final part of the thesis focuses on systematic loss analyses. Through transfer-matrix method (TMM)-based simulations, optical losses can be expressed in terms of the corresponding Jsc losses: The front IZO and C60 layers represent a source of parasitic absorption (losses amount to 1.2 mA cm−2) and strong reflection losses are observed between 450- 500 nm. The optical loss analyses are then complemented with a systematic analysis of nonradiative recombination losses. By absolute photoluminescence (PL) measurements, the C60/CH3NH3PbI3 (MAPI) interface results to be a significant source of Voc losses (the QFLS decreases from 1.23 eV for the bare MAPI to 1.10 eV for MAPI grown on C60). To reduce these losses different vacuum deposited interlayers (BCP, B4PyMPM, 3TPYMB and LiF) are screened. An improvement in QFLS of 30-40 meV is observed with respect to C60/MAPI heterojunction by employing B4PyMPM, 3TPYMB and LiF. Their further implementation in flexible PSCs support the conclusions from absolute PL, observing a comparable improvement in Voc values when these interlayers are applied, confirming their passivating role against non-radiative recombination. Further investigations on MAPI/hole transport layer (HTL) interface point out the detrimental role of dopants in Spiro-OMeTAD film (widely employed HTL in the community) as recombination centers upon oxidation and light exposure. Show more
External linksSearch print copy at ETH Library
ContributorsExaminer: Tiwari, Ayodhya N.
Examiner: Wood, Vanessa
Examiner: Snaith, Henry J.
Examiner: Bücheler, Stephan
Subjectphotovoltaics; perovskite solar cell; flexible solar cell; thin film; tandem photovoltaics
Organisational unit02140 - Dep. Inf.technologie und Elektrotechnik / Dep. of Inform.Technol. Electrical Eng.
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