A systematic research on rational design and synthesis of innovative materials for developing high-performance perovskite solar cells

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2023-6-13

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Karabağ, Aliekber

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Solution-processable perovskite solar cells (PSCs) has been a rising solar cell revolutionary since the beginning of first prototype in 2009, and up till now the certified photovoltaic device efficiencies have gone beyond the 26% threshold. To attain highly efficient and durable PSCs, it is critical to develop sophisticated materials for essential functional layers. Rationally designed hole transporting materials (HTMs), that is utilized as hole transport layers (HTLs) serve as one of the key fragments used for boosting the hole extraction and transportation, and thereby enhancing the efficiency and the long-term durability of fabricated PSCs. Currently, 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (Spiro-OMeTAD) represents the benchmark HTM in the state-of-the-art PSCs, however it also suffers from tedious synthesis process, high-price, and the requirement of dopant/additive combination which directly prevents the commercialization of cells that utilize this HTM. In this manner, its utilization has not gone beyond from being a reference HTM in the control device. Currently, novel HTMs with facile synthetic pathways, favoured optoelectronic properties, well-matched energy levels, cost-effectiveness and fabrication compatibility for high-performance PSCs is still in urgent demand. In this thesis, a set of innovative HTMs were synthesized via traditional Pd-catalyzed cross couplings (Stille and Suzuki-Miyaura) and their performance in PSCs were evaluated. 6 unique organic-, and 11 different polymeric-based HTMs (org-HTM2-3, 5-6, 8-9; poly-HTM1a, 1b, 2a, 2b, 2c, 3a, 3b, 3d, 4a, 4b, 4c); hence a small HTM collection, were successfully synthesized. In material design protocols, organic HTMs (org-HTM1-9) were aimed to be functionalized by integrating the same units of electron-rich triphenylamine derivative (p-OMe-TPA) as the pendant unit and five-membered-ring aromatic heterocyclics (furan, thiophene, and selenophene) as a π-linker to raise the conductivity and the charge transportation capability of HTMs on different central core skeletons such as electron-rich 4H-dithieno[3,2-b:2’,3’-d]pyrrole (DTP)-, and 9H-fluorene (FLUO)-based donor units and electron-deficient 4H-thieno[3,4-c]pyrrole-4,6-dione (TPD)-based acceptor unit. Also, these frameworks were modified through the central core skeletons with the same long linear carbon chain to increase the solubility of resultant HTMs. On the other hand, polymeric HTMs were based on diketopyrrolopyrrole (DPP) accepotor core and abovementioned furan and thiophene π-linkers utilized with selenophene-based donor units and different carbon alkyl chains. All strategies, applied for the syntheses of both organic and polymeric HTMs (org-HTMs: D-π-D-π-D, D-π-A-π-D; poly-HTMs: -π-A-π-D-) were aimed to gain further functionality of developed HTMs such as tuning their HOMO energy levels to be compatible with corresponding perovskites and boosting their conductivities and charge transportation abilities. The photovoltaic device performances of each organic and polymeric HTMs were systematically examined in n-i-p structured PSCs. Both their pristine and doped versions were studied by deposition atop either double cation (FAPbI3)1-x(MAPbBr3)x-based (1st set of org-HTMs: org-HTM2-3) or triple cation (Cs0.04FA0.85MA0.11)Pb(I0.96Br0.01Cl0.03)3-based (2nd set of org-HTMs: org-HTM2-3; 3rd set of org-HTMs: org-HTM5-6, 8-9) perovskite layers. As a final study, long-term stability of doped org-HTM2-3, 5-6, 8-9 in the presence of Li-TFSI, FK209, and t-BP were analyzed under humid environment. The 1st and 2nd sets of cells contained the same organic HTMs, namely org-HTM2-3 and differed from each other in terms of perovskite composition and functional layers. Among them, dopant-free org-HTM2 (15.55%) and org-HTM3 (15.16%) showed quite similar results in terms of device efficiencies when they are deposited on top of FAMA-based perovskite layer. Despite demonstrating higher efficiency PSCs than dopant-free Spiro-OMeTAD HTM-based control device (11.83%), the results did not exceed the critical 20% threshold. Due to slightly higher device efficiency of org-HTM2, the next PSC study was conducted by doping this HTM and achieved device efficiencies (17.56%) that are comparable but lower than the doped Spiro-OMeTAD HTM-based the control device (20.86%). Thereby, as a further optimization, DTP-based org-HTM2-3 HTMs were tested in PSCs, having CsFAMA-based perovskite composition and commonly-utilized c-TiO2 ETM and PEAI passivation layer. The 2nd set of doped HTMs (org-HTM2: 18.83%; org-HTM3: 19.20%) showed better photovoltaic device efficiencies compared to the 1st set of doped HTMs and almost at the same level of the control device efficiency (20.37%). The abovementioned preliminary results show that small changes such as modifications on perovskite composition and device configuration have a great impact on the device performance. It is highly believed that making further optimizations will lead to unleash the full potential of org-HTM2-3 in constructing high-performance PSCs. Within the 3rd set of PSC batches, selenophene-containing TPD-modified device (org-HTM6) achieved the record photovoltaic device efficiency (21.92%), higher than the control device efficiency (21.22%). The switch to thiophene π-linker (org-HTM5: 15.85%) had a detrimental effect on device efficiency compared to selenophene analogue (org-HTM6). When compared to the same class of FLUO-based organic HTMs (org-HTM8-9), thiophene-containing HTM (org-HTM8: 18.85%) exhibited similar device efficiency with selenophene-containing HTM (org-HTM9: 18.69%). Based on all these initial results, it is concluded that novel org-HTM6 have a great potential to replace the universal Spiro-OMeTAD HTM. Additionally, all findings prove that D-A-D type organic HTMs are highly promising p-type materials than the D-D-D type HTMs to give higher device performances in PSCs. The stability of the PSCs containing doped organic HTMs in the 2nd and 3rd sets that showed the best performance were tested. All resultant PSCs were prepared without encapsulation and stored under ambient air, with a relative humidity (RH) of around 15% at dark environment, at room temperature with various aging times. Based on the long-term stability results, achieved by 2nd set of HTMs, org-HTM2-treated PSC retained its initial PCE unchanged for 528 h. On the other hand, a slight change in PCE for org-HTM3-based PSC (10% loss) were observed compared to Spiro-OMeTAD (4% loss of PCE) under same condition. The champion device, containing org-HTM6 also showed outstanding stability (13% PCE loss for 3000 h) whereas org-HTM5-based PSC showed quite similar stability (18% loss of PCE). As it is expected, Spiro-OMeTAD-based control device degraded too much (32% loss of PCE) under the same environment. FLUO-cored org-HTM8-9 also exhibited promising stability, especially selenophene-containing org-HTM9 (5% loss of PCE) compared to that of thiophene-containing org-HTM8 (12% loss of PCE) and Spiro-OMeTAD-based control device (11% loss of PCE). The findings confirm that, the insertion of selenophene π-linker into both TPD- and FLUO-modified organic HTMs rather than thiophene analogue considerably improves the durability of the fabricated PSCs. An opposite trend is valid for DTP-modified organic HTMs. Since the type of both π-linker and the central core unit on these types of molecular structures have a notable impact on both device efficiency and the long-term stability of the constructed PSCs, considering this information will greatly help to develop high performance novel organic HTMs based on DTP, TPP, and FLUO skeletons. Within polymeric HTM-treated PSCs, furan-modified frameworks with a linear alkyl chain (poly-HTM3d: 8.55%) showed higher photovoltaic device efficiencies compared to branch alkyl structures (poly-HTM3b: 6.75%) in PSCs whereas an opposite trend was observed for thiophene-modified analogue (linear poly-HTM4a: 11.19%; branch poly-HTM4b: 11.80%; branch poly-HTM4c: 15.35%). Also, the DPP polymers based on selenophene donor unit showed better device efficiencies than the 2,2’-biselenophene-based DPP polymers except for the trend between poly-HTM2c (13.33%) and poly-HTM4c (15.35%). It is also proved that the nature of π-linker on polymeric backbones have an important impact on device efficiencies. The thiophene-containing polymeric HTMs showed better photovoltaic device efficiencies compared to the furan counterparts when the type of alkyl chain and donor units were kept same on the molecular structure. For instance, poly-HTM4a and poly-HTM4b (11.80%) outperformed the poly-HTM3a (6.60%) and poly-HTM3b in PSC studies. With a few optimizations, it is highly believed that these preliminary results based on DPP-modified polymeric HTMs (around 16%) will be enhanced to levels that can compete with the efficiency of the DPP materials (almost 19%) in the literature. In the final part of the thesis, a robust, three-step, novel method was developed for synthesis of large ammonium cations that are re required for realizing 2D-Perovskite material. 2D-Perovskite shows significantly enhances stabilities compared to their 3D-counterparts albeit with much lower efficiencies. To get the best of both worlds, 3D/2D perovskites are commonly utilized in the literature with enhanced efficiencies and stabilities. Even though great success was achieved with this approach, no systematic study on the effect of electronics and steric of the large ammonium cations have been investigated. By utilizing our facile method, mainly two different types of multi-functional large organic cations (aromatic heterocyclic 2-C4H3X-EA+ where X: O, S, Se; aromatic x-XPEA+ where x: o, m, p and X: F, Cl, Br) with an iodide (I-) counter ion (total 12 salts) were synthesized and tested in n-i-p type PSCs. Among large bulky organic cations, heterocyclic ethylammonium iodide (2-C4H3X-EAI) salts were utilized as a passivation layer on top of triple-cation CsFAMA-based 3D perovskite layer whereas all the halogenated phenylethylammonium iodide (x-XPEAI) salts were deposited as 2D capping layer on top of the same perovskite layer. Thiophene-containing 2-THIO-EAI (19.50%) showed higher efficiency than 2-FRN-EAI (14.90%) and 2-SELO-EAI (18.00%) due to better passivation of 2-THIO-EAI salt, and comparable efficiency with the control device (19.60%). It is strongly expected that further optimizations will raise the device performance beyond 20% threshold. All these findings indicate that utilization of five-membered-ring structured of organic salts as a passivation layer, deposited on top of 3D-PL have a promising strategy to develop high performance PSCs. Within the phenylethylammonium iodide salts, independent of the substituent type, -meta positioned m-XPEAI salts remarkably outperformed their -ortho and -para counterparts. Among -meta positioned 2D salts (m-XPEAI), m-BrPEAI (23.42%) gave the best performing device efficiency, resulting in only slightly higher PCE than the m-FPEAI (23.16%) and m-ClPEAI (23.08%). It should be noted that the treatment of all PEAI-based salts significantly enhances the device efficiency (PCE of 20.80%). Owing to the remarkable hydrophobicity of halogen-substituted phenylethylammonium-based salts displayed superior moisture resistance and retained 97% initial PCE after aging at 200 days (over 20% relative humidity (RH), 20-25 °C) compared to the control device (reduced to 86%). These salts also showed great thermal stability after aging 1800 s (60-70% RH, 60 °C) compared to the control device (decreased to 73%). Overall, m-ClPEAI salt leaded to the record device performance in 3D/2D PSCs when all efficiency, stability and reproducibility aspects are all taken into account. All findings related to not only organic and polymeric hole transport materials but also large bulky organic cations, achieved in this thesis indicate the significant role of core skeletons, π-linkers, side chains, substituents, and their positions on rationally designed molecular structures. It is highly believed that all abovementioned results accomplished from these comprehensive studies will provide useful guidelines in designing high performance HTMs and multi-functional large bulky organic cations in PSCs.

Subject Keywords

Material design and synthesis, Perovskite solar cells (PSCs), Photovoltaic device efficiency and stability, Hole transport materials (HTMs), Large bulky organic cations

URI

https://hdl.handle.net/11511/104495

Collections

Graduate School of Natural and Applied Sciences, Thesis