Kyung Taek Cho

Highly efficient perovskite solar cells with a compositionally engineered perovskite/hole transporting material interface

Perovskite solar cells (PSCs) have experienced an outstanding advance in power conversion efficiency (PCE) by optimizing the perovskite layer morphology, composition, interfaces, and charge collection efficiency. To enhance PCE, here we developed a new method i.e., engineering a compositional gradient thinly at the rear interface between the perovskite and the hole transporting materials. We demonstrate that charge collection is improved and charge recombination is reduced by formation of an engineered passivating layer, which leads to a striking enhancement in open-circuit voltage (VOC). The passivation effect induced by constructing an additional FAPbBr3−xIx layer on top of the primary (FAPbI3)0.85(MAPbBr3)0.15 film was proven to function as an electron blocking layer within the perovskite film, resulting in a final PCE of 21.3%. Our results shed light on the importance of the interfacial engineering on the rear surface of perovskite layers and describe an innovative approach that will further boost the PSC efficiency.

Enhanced Charge Collection with Passivation Layers in Perovskite Solar Cells

The Al2 O3 passivation layer is beneficial for mesoporous TiO2 -based perovskite solar cells when it is deposited selectively on the compact TiO2 surface. Such a passivation layer suppressing surface recombination can be formed by thermal decomposition of the perovskite layer during post-annealing.

Molecular engineering of face-on oriented dopant-free hole transporting material for perovskite solar cells with 19% PCE

Through judicious molecular engineering, novel dopant-free star-shaped D–π–A type hole transporting materials coded KR355, KR321, and KR353 were systematically designed, synthesized and characterized. KR321 has been revealed to form a particular face-on organization on perovskite films favoring vertical charge carrier transport and for the first time, we show that this particular molecular stacking feature resulted in a power conversion efficiency over 19% in combination with mixed-perovskite (FAPbI3)0.85(MAPbBr3)0.15. The obtained 19% efficiency using a pristine hole transporting layer without any chemical additives or doping is the highest, establishing that the molecular engineering of a planar donor core, π-spacer and periphery acceptor leads to high mobility, and the design provides useful insight into the synthesis of next-generation HTMs for perovskite solar cells and optoelectronic applications.

Dopant-Free Hole-Transporting Materials for Stable and Efficient Perovskite Solar Cells

Molecularly engineered novel dopant-free hole-transporting materials for perovskite solar cells (PSCs) combined with mixed-perovskite (FAPbI3 )0.85 (MAPbBr3 )0.15 (MA: CH3 NH3+ , FA: NH=CHNH3+ ) that exhibit an excellent power conversion efficiency of 18.9% under AM 1.5 conditions are investigated. The mobilities of FA-CN, and TPA-CN are determined to be 1.2 × 10-4 cm2 V-1 s-1 and 1.1 × 10-4 cm2 V-1 s-1 , respectively. Exceptional stability up to 500 h is measured with the PSC based on FA-CN. Additionally, it is found that the maximum power output collected after 1300 h remained 65% of its initial value. This opens up new avenue for efficient and stable PSCs exploring new materials as alternatives to Spiro-OMeTAD.

A highly hindered bithiophene-functionalized dispiro-oxepine derivative as an efficient hole transporting material for perovskite solar cells

Dimethoxydiphenylamine-substituted dispiro-oxepine derivative 2,2′,7,7′-tetrakis-(N,N′-di-4-methoxyphenylamine)dispiro-[fluorene-9,4′-dithieno[3,2-c:2′,3′-e]oxepine-6′,9′′-fluorene] (DDOF) has been designed and synthesized using a facile synthetic route. The novel hole transporting material (HTM) was fully characterized and tested in perovskite solar cells exhibiting a remarkable power conversion efficiency of 19.4%. More importantly, compared with spiro-OMeTAD-based devices, DDOF shows significantly improved stability. The comparatively comprehensive solid structure study is attempted to disclose the common features of good performance HTMs. These achievements clearly demonstrated that the highly hindered DDOF can be an effective HTM for the fabrication of efficient perovskite solar cells and further enlightened the rule of new HTMs design.

An efficient perovskite solar cell with symmetrical Zn(II) phthalocyanine infiltrated buffering porous Al2O3 as the hybrid interfacial hole-transporting layer

A new Zn(ii) phthalocyanine (Pc) based low bandgap HTM is introduced for perovskite solar cells. Steady state and time-resolved photoluminescence (PL) measurements indicated an evenly matched hole extraction efficiency between sym-HTPcH and spiro-OMeTAD. On account of the low film quality and resulting high recombination, Zn(ii) Pc normally cannot work as an effective HTM. We adopted insulating Al2O3 for the infiltration of sym-HTPcH to form a hybrid interfacial buffer layer, affording perovskite solar cells (PSCs) with an average PCE value of up to 12.3%, which is a significant improvement with respect to the control cell without the meso-Al2O3 layer (4.21%) and is the highest value ever reported for Zn(ii) phthalocyanine based devices under AM1.5G standard conditions. A hysteresis test revealed that our device structure with the new HTM exhibited a balanced charge extraction behaviour.

Beneficial Role of Reduced Graphene Oxide for Electron Extraction in Highly Efficient Perovskite Solar Cells

In this work we systematically investigated the role of reduced graphene oxide (rGO) in hybrid perovskite solar cells (PSCs). By mixing rGO within the mesoporous TiO2 (m-TiO2 ) matrix, highly efficient solar cells with power conversion efficiency values up to 19.54 % were realized. In addition, the boosted beneficial role of rGO with and without Li-treated m-TiO2 is highlighted, improving transport and injection of photoexcited electrons. This combined system may pave the way for further development and optimization of electron transport and collection in high efficiency PSCs.

Enhanced charge collection with passivation of the tin oxide layer in planar perovskite solar cells

Tin oxide is an excellent candidate to replace mesoporous TiO2 electron transport layers (ETLs) in perovskite solar cells. Here, we introduced a SnO2 layer by a low-temperature solution process, and investigated its morphology, opto-physical and electrical properties affecting the device performance. We reveal that low-temperature processed SnO2 is self-passivating in nature, which leads to a high efficiency. To further enhance the blocking effect, we combined a compact TiO2 underlayer with the SnO2 contact layer, and found that the bi-layered ETL is superior compared to single layers. The best device shows photovoltaic values in a planar structure with a short-circuit current density (Jsc) of 22.58 mA cm−2, an open-circuit voltage (Voc) of 1.13 V, a fill factor (FF) of 0.78, and a power conversion efficiency (PCE) of 19.80% under 1 sunlight illumination.

Selective growth of layered perovskites for stable and efficient photovoltaics

Perovskite solar cells (PSCs) are promising alternatives toward clean energy because of their high-power conversion efficiency (PCE) and low materials and processing cost. However, their poor stability under operation still limits their practical applications. Here we design an innovative approach to control the surface growth of a low dimensional perovskite layer on top of a bulk three-dimensional (3D) perovskite film. This results in a structured perovskite interface where a distinct layered low dimensional perovskite is engineered on top of the 3D film. Structural and optical properties of the stack are investigated and solar cells are realized. When embodying the low dimensional perovskite layer, the photovoltaic cells exhibit an enhanced PCE of 20.1% on average, when compared to pristine 3D perovskite. In addition, superior stability is observed: the devices retain 85% of the initial PCE stressed under one sun illumination for 800 hours at 50 °C in an ambient environment.

Femtosecond Charge-Injection Dynamics at Hybrid Perovskite Interfaces

With a power conversion efficiency (PCE) exceeding 22 %, perovskite solar cells (PSCs) have thrilled photovoltaic research. However, the interface behavior is still not understood and is a hot topic of research: different processes occur over a hierarchy of timescales, from femtoseconds to seconds, which makes perovskite interface physics intriguing. Herein, through femtosecond transient absorption spectroscopy with spectral coverage extending into the crucial IR region, the ultrafast interface-specific processes at standard and newly molecularly engineered perovskite interfaces in state-of-the-art PSCs are interrogated. Ultrafast interfacial charge injection occurs with a time constant of 100 fs, resulting in hot transfer from energetic charges and setting the timescale for the first step involved in the complex charge-transfer process. This is also true for 20 % efficient devices measured under real operation, for which the femtosecond injection is followed by a slower picosecond component. These findings provide compelling evidence for the femtosecond interfacial charge-injection step and demonstrate a robust method for the straightforward identification of interfacial non-equilibrium processes on the ultrafast timescale.