Perovskite solar cell stability under long term operation

Ajdič, Žan (author), Jošt, Marko (mentor)

Perovskite solar cells are a novel thin-film photovoltaics technology that has gained lots of attention over the last 2 decades. Following extensive device optimization, power conversion efficiency (PCE) now rivals the one of the dominating silicon technology. However, despite their exceptional performance potential, the commercialization of perovskite solar cells remains limited by insufficient long-term stability. The ionic nature of perovskites makes them susceptible to degradation under environmental stressors such as moisture, oxygen, light, and elevated temperatures. Therefore, improving the operational stability and lifetime of PSCs has become a primary research challenge, especially as efficiencies approach theoretical limits. This doctoral research focuses on the p‑i‑n configuration of perovskite solar cells using a formamidinium–cesium‑based (FACs) mixed‑halide perovskite absorber. The FACs devices are known for their robustness and were thus chosen as the main composition tested, focusing on an absorber with the chemical formula FA0.83Cs0.17Pb(I0.83Br0.17)3. The perovskite solar cells had an inverted (p-i-n) planar structure and a layer configuration of glass/ITO/MeO-2PACz/FACs/C60/SnO2/Cu. Overall more than 1500 devices was fabricated with reference devices demonstrating PV performance parameters with median values of open‑circuit voltage (VOC) at 1.09 V, short‑circuit current density (JSC) at 19.9 mA/cm², fill factor (FF) at 74.5%, and power conversion efficiency (PCE) at 16.4%. Champion devices have exceeded 18%. The core of the doctoral dissertation are three published papers. The first one analyzes the role of a thin Al₂O₃ layer deposited by atomic layer deposition. It is investigated both as an ultrathin interfacial passivation layer at three relevant interfaces and as a moisture-blocking capping layer. The influence of Al₂O₃ thickness is systematically studied at the perovskite/C60, C60/SnO2, and SnO2/Cu interfaces. Ultrathin Al₂O₃ layers (1–2 nm) at the perovskite/C60 and C60/SnO2 interfaces improve cell efficiency and water resistance, respectively, while deposition at the SnO2/Cu interface results in an S-shaped J-V curve. When applied as a 30 nm thick capping layer, Al₂O₃ significantly improves resistance to moisture ingress without compromising device efficiency after recovery from high-temperature exposure during the ALD process. The improvement against water ingress is evaluated by visually monitoring degradation when cells are immersed in water. Furthermore, with the capping layer, long-term stability improves at least fourfold compared to non-capped devices. To further enhance long-term stability, Parylene F‑VT4 polymer is investigated as an additional encapsulation barrier. We test three different thicknesses: 2, 5, and 15 µm. Combined Al2O3/parylene encapsulation substantially improves resistance to moisture and prolongs operational lifetimes, with an optimal parylene thickness of 5 µm, which performs best in all tests: water test, damp-heat test, and long-term stability. Compared to the reference capped devices, stability improves by 1350%, 1000%, and 300%, respectively. Excessively thick polymer coatings were found to delaminate and fail mechanically. The Parylene results were also published in a scientific journal. The third paper analyzes in detail long-term stability of perovskite solar cells. For this, a custom white LED (WLED) testing system was developed in the laboratory. This setup enables simultaneous maximum power point tracking (MPPT) of up to 144 solar cells. Stability is quantified using the t80 metric, defined as the time required for the output power to decline to 80% of its maximum value. A comprehensive statistical analysis was conducted on more than 500 perovskite solar cells to ensure robust conclusions beyond single-device demonstrations. We examine the effects of moisture, light intensity, electrical bias voltage, and perovskite composition, particularly the effect of bromide content. In the long-term MPP tracking tests, we first confirm that light is the primary degradation contributor by performing cyclic tests and tests under different light intensities. Photoluminescence measurements reveal photo-induced halide phase segregation in mixed-halide perovskites, leading to band gap redshift and accelerated degradation. Devices without bromide ions exhibit significantly improved stability, with t80 lifetimes increased by up to a factor of five. We also note that during the aging process, perovskite usually degrades faster in electrically inactive areas, where the perovskite is effectively held in open-circuit condition. We show that there is a linear correlation between t80 time and the bias voltage applied to the cell during stability tracking. By testing a large number of samples, we demonstrate with statistical significance that long-term stability measurements show significantly higher spread (both batch-to-batch and intra-batch) than J-V measurements. In the final content chapter, we investigate the effects of different materials used for selected layers, such as the hole transport layer and back contacts, as well as the effects of process or fabrication parameters, including ITO thickness, the type of antisolvent used during perovskite spin-coating deposition, and the amount of PbI2 excess in the perovskite composition. We confirm beneficial effects on long-term stability by switching the back electrode material from copper to silver and by using a different antisolvent (anisole or toluene) instead of typically used ethyl acetate. Too much PbI2 excess accelerates degradation, while introducing a new monolayer – 4PADCB – increases PCE by 1%; however, in some cases, it reduces long-term stability. The results are also not conclusive for the ITO thickness therefore some of the experiments will be repeated together with performing additional test further exploring the effect of cell structure and process parameters on stability. Overall, this doctoral research demonstrates that stable, high-efficiency perovskite solar cells can be achieved through interface engineering, careful material selection, and advanced encapsulation strategies. The systematic, statistically robust approach presented here offers valuable insights into degradation mechanisms and practical methods for extending the operational lifetime of perovskite solar cells, bringing this promising technology closer to commercial viability.

• Language: English
• Keywords: perovskite solar cells, long term stability, Al2O3, parylene, moisture, light, statistical analysis, maximum power point tracking
• Work type: Doctoral dissertation
• Organization: FE - Faculty of Electrical Engineering
• Year: 2026
• PID: 20.500.12556/RUL-189302
• Publication date in RUL: 02.10.2026

Secondary language

• Language: Slovenian
• Title: Stabilnost perovskitnih sončnih celic med dolgoročnim delovanjem

Abstract

Perovskitne sončne celice spadajo v tretjo generacijo sončnih celic, ki je v zadnjih dveh desetletjih pritegnila veliko pozornosti. Njihova učinkovitost pretvorbe energije (PCE) danes dosega raven prevladujoče silicijeve tehnologije, vendar njihovo komercializacijo še vedno omejuje nezadostna dolgoročna stabilnost. Zaradi ionske narave perovskitov so ti materiali občutljivi na vplive okolja, kot so vlaga, kisik, svetloba in visoka temperatura. Zato je izboljšanje obratovalne stabilnosti perovskitnih sončnih celic postalo eden glavnih raziskovalnih izzivov, zlasti ker se njihove učinkovitosti približujejo teoretičnim mejam. Doktorska raziskava se osredotoča na perovskitne sončne celice v konfiguraciji p-i-n z absorberjem na osnovi tipičnega FACs perovskita s kemijsko formulo FA0.83Cs0.17Pb(I0.83Br0.17)3. Celice FACs so znane po svoji robustnosti, zato so bile izbrane kot osrednji predmet raziskave. Analizirane sončne celice imajo invertirano (p-i-n) planarno strukturo s konfiguracijo plasti steklo/ITO/MeO-2PACz/FACs/C60/SnO2/Cu. Skupno je bilo izdelanih več kot 1500 celic. Referenčne celice so dosegle naslednje mediane vrednosti fotovoltaičnih parametrov: napetost odprtih sponk (VOC) 1,09 V, gostoto kratkostičnega toka (JSC) 19,9 mA/cm2, faktor polnitve (FF) 74,5 % in učinkovitost pretvorbe energije (PCE) 16,4 %. Osrednji del doktorske disertacije predstavljajo trije objavljeni znanstveni članki. Prvi obravnava vpliv tankih plasti Al2O3, nanesene z metodo atomske depozicije (ALD). Raziskana je bila njena uporaba kot ultratanka pasivacijska plast na treh izbranih spojih ter kot zaščitna pokrivna plast za preprečevanje vdora vlage. Sistematično smo preučili vpliv debeline Al2O3 na spojih perovskit/C60, C60/SnO2 in SnO2/Cu. Ugotovili smo, da ultratanke plasti Al2O3 (1 do 2 nm) na spojih perovskit/C60 in C60/SnO2 izboljšajo učinkovitost oziroma odpornost proti vodi, medtem ko nanos na spoj SnO2/Cu povzroči opazen upad PCE. V primeru uporabe Al2O3 kot 30 nm debela pokrivna plast, se bistveno izboljšal odpornost proti vdoru vlage pri čemer se vrednost PCE ohrani. Izboljšanje odpornosti proti vdoru vode smo ovrednotili z vizualnim spremljanjem degradacije celic, potopljenih v vodo. Poleg tega se je dolgoročna stabilnost celic s pokrivno plastjo izboljšala za najmanj štirikrat v primerjavi z nezaščitenimi celicami. Za dodatno izboljšanje dolgoročne stabilnosti smo preučili uporabo polimera parilea F-VT4 kot dodatnega zaščitnega sloja. Raziskali smo tri različne debeline plasti: 2, 5 in 15 µm. Kombinirana zaščita Al2O3/parilen je občutno izboljšala odpornost proti vlagi in podaljšala življenjsko dobo celic. Kot optimalna se je izkazala debelina 5 µm, ki je dosegla najboljše rezultate pri vseh izvedenih testih: testu potopitve v vodo, testu v okoljski komori in testu dolgoročne stabilnosti . V primerjavi z referenčnimi celicami, zaščitenimi samo z Al2O3, se je stabilnost izboljšala za 1350 %, 1000 % oziroma 300 %. Ugotovili smo tudi, da predebele plasti polimera povzročajo mehanske težave zaradi delaminacije. Tretji članek podrobno obravnava dolgoročno stabilnost perovskitnih sončnih celic. V ta namen je bil v laboratoriju razvit namenski sistem z belimi LED-diodami (WLED), ki omogoča sočasno sledenje točki maksimalne moči (MPPT) za 144 sončnih celic. Stabilnost smo vrednotili z metriko t80, ki predstavlja čas, potreben, da moč celice pade pod 80 % svoje največje vrednosti. Na več kot 500 perovskitnih sončnih celicah smo izvedli obsežno statistično analizo, ki omogoča zanesljive zaključke na podlagi velikega števila vzorcev. Analizirali smo vpliv vlage, intenzitete svetlobe, električne napetosti in kompozicije perovskita. Z dolgoročnimi MPPT-meritvami smo potrdili, da je svetloba glavni dejavnik degradacije. Meritve fotoluminiscence so razkrile svetlobno inducirano fazno segregacijo halidnih ionov v perovskitu, kar povzroča zmanjšanje energijske reže in pospešeno degradacijo. Celice brez bromidnih ionov so dosegle tudi do petkrat daljše življenjske čase t80. Opazili smo tudi, da perovskit med staranjem hitreje degradira na električno neaktivnih območjih, kjer je material v režimu odprtih sponk. Pokazali smo linearno povezavo med časom t80 in napetostjo, aplicirano na celico med testiranjem stabilnosti. Testiranje velikega števila vzorcev je statistično potrdilo, da meritve dolgoročne stabilnosti izkazujejo velik raztros vrednosti, zaradi česar je pomembno poročanje popolnih podatkov, ne le rezultatov najboljših naprav. V zadnjem poglavju smo raziskali vpliv različnih materialov, uporabljenih v posameznih plasteh sončne celice ter vpliv procesnih parametrov izdelave na dolgoročno stabilnost. Ugotovili smo pozitiven vpliv zamenjave bakrenih zadnjih elektrod s srebrnimi ter uporabe anizola ali toluena namesto običajno uporabljenega etil acetata. Po drugi strani prevelik presežek PbI2 pospešuje degradacijo celic. Uporaba nove molekule v funkciji plasti za transport vrzeli (4PADCB) poveča učinkovitost pretvorbe za približno 1 %, vendar lahko v določenih primerih poslabša dolgoročno stabilnost. Rezultati glede vpliva debeline ITO za zdaj še niso dokončni, zato bodo nekateri eksperimenti ponovljeni in dopolnjeni z dodatnimi analizami vpliva strukture celice in procesnih parametrov na stabilnost. Doktorsko delo dokazuje, da je mogoče z ustreznim inženiringom spojev, premišljeno izbiro materialov in naprednimi metodami enkapsulacije izdelati stabilne in visoko učinkovite perovskitne sončne celice. Sistematičen, statistično podprt pristop omogoča boljše razumevanje degradacijskih mehanizmov ter ponuja praktične rešitve za podaljšanje življenjske dobe teh naprav. Predstavljeni rezultati pomembno prispevajo k približevanju perovskitnih sončnih celic komercialni uporabi in njihovi širši vključitvi v prihodnje fotovoltaične sisteme.

• Keywords: perovskitne sončne selice, dolgoročna stabilnost, Al2O3, parilen, vlaga, svetloba, statistična analiza, sledenje točki maksimalne moči