Selenophene-Based Hole-Transporting Materials for Perovskite Solar Cells

Two novel and simple donor-pi-bridge-donor (D-pi-D) hole-transporting materials (HTMs) containing two units of the p-methoxytriphenylamine (TPA) electron donor group covalently bridged by means of the 3,4-dimethoxyselenophene spacer through single and triple bonds are reported. The optoelectronic and thermal properties of the new selenium-containing HTMs have been determined using standard experimental techniques and theoretical density functional theory (DFT) calculations. The selenium-based HTMs have been incorporated in mesoporous perovskite solar cells (PSCs) in combination with the triple-cation perovskite (FAPbI(3))(0.87)(MAPbBr(3))(0.13) CsPbI3. Limited values of power conversion efficiencies, up to 13.4 %, in comparison with the archetype spiro-OMeTAD (17.8 %), were obtained. The reduced efficiencies showed by the new HTMs are attributed to their poor film-forming ability, which constrains their photovoltaic performance due to the appearance of structural defects (pinholes).

Strategy to Boost the Efficiency of Mixed-Ion Perovskite Solar Cells: Changing Geometry of the Hole Transporting Material

Ulf Anders Hagfeldt, Nikolaos Vlachopoulos, Jiahuan Zhang

The hole transporting material (HTM) is an essential component in perovskite solar cells (PSCs) for efficient extraction and collection of the photoinduced charges. Triphenylamine- and carbazole-based derivatives have extensively been explored as alternative and economical HTMs for PSCs. However, the improvement of their power conversion efficiency (PCE), as well as further investigation of the relationship between the chemical structure of the HTMs and the photovoltaic performance, is imperatively needed. In this respect, a simple carbazole-based HTM X25 was designed on the basis of a reference HTM, triphenylamine-based X2, by simply linking two neighboring phenyl groups in a triphenylamine unit through a carbon-carbon single bond. It was found that a lowered highest occupied molecular orbital (HOMO) energy level was obtained for X25 compared to that of X2. Besides, the carbazole moiety in X25 improved the molecular planarity as well as conductivity property in comparison with the triphenylamine unit in X2. Utilizing the HTM X25 in a solar cell with mixed-ion perovskite HC(NH2)(2)(CH3NH3)(0.15)Pb(I0.85Br0.15)(3), a highest reported PCE of 17.4% at 1 sun (18.9% under 0.46 sun) for carbazole-based HTM in PSCs was achieved, in comparison of a PCE of 14.7% for triphenylamine-based HTM X2. From the steady-state photoluminescence and transient photocurrent/photovoltage measurements, we conclude that (1) the lowered HOMO level for X25 compared to X2 favored a higher open-circuit voltage (V-oc) in PSCs; (2) a more uniform formation of X25 capping layer than X2 on the surface of perovskite resulted in more efficient hole transport and charge extraction in the devices. In addition, the long-term stability of PSCs with X25 is significantly enhanced compared to X2 due to its good uniformity of HTM layer and thus complete coverage on the perovskite. The results provide important information to further develop simple and efficient small molecular HTMs applied in solar cells.

Amer Chemical Soc2016

Molecularly Engineered Hole Transporting Materials for High Performance Perovskite Solar Cells

Kasparas Rakstys

Perovskite solar cells have rapidly revolutionized the photovoltaic research showing an im-pressively dynamic progress on power conversion efficiency from 3.8 to 22% in only several years, a record for a nascent technology. Furthermore, inexpensive precursors and simple fabrication methods of perovskite materials hold a great potential for future low-cost energy generation enabling the global transition to a low-carbon society. The best performing device configuration of perovskite solar cell is composed of an electron transporting material, typi-cally a mesoporous layer of titanium dioxide, which is infiltrated with perovskite material and coated with a hole transporting material. However, although perovskite solar cells have achieved high power conversion efficiency values, there are several challenges limiting the industrial realization of low-cost, stable, and high-efficiency photovoltaic devices. To date, spiro-OMeTAD and PTAA are hole transporting materials of choice in order to main-tain the highest efficiency, however, the prohibitively high price hinders progress towards cheap perovskite solar cell manufacturing and may contribute to more than 30% of the overall module cost. Additionally, such wide bandgap hole transporting materials typically require doping in order to match necessary electrical conductivity and the use of additives is prob-lematic, since hygroscopic nature of doping makes the hole transporting layer highly hydro-philic leading to rapid degradation, negatively influencing the stability of the entire device. In order to overcome these problems, the rational design, synthesis, and characterization of a variety of small molecule-based hole transporting materials have been on a focus of this the-sis. Through judicious molecular engineering four innovative hole transporting materials KR131, KR216, KR374, and DDOF were developed via alternative synthetic schemes with the minimized number of steps and simple workup procedures allowing cost-effective upscale. Employing various characterization methods, the relationship between the molecular struc-ture of the novel hole transporting materials and performance of perovskite solar cells was investigated, leading to a fundamental understanding of the requirements of the hole trans-porting materials and further improvement of the photovoltaic performance. Furthermore, the synthesis of the dopant-free hole transporting materials based on push-pull architecture is presented. Highly ordered characteristic face-on organization of KR321 hole transporting molecules benefits to increased vertical charge carrier transport within a perov-skite solar cell, leading to a power conversion efficiency over 19% with improved durability. The obtained result using pristine hole transporting material is the highest and outperforms most of the other dopant-free hole transporting materials reported to date. Highly hydropho-bic nature of KR321 may serve as a protection of perovskite layer from the moisture and pre-vent the diffusion of external moieties, showing a promising avenue to stabilize perovskite solar cells.

EPFL2018

Comprehensive control of voltage loss enables 11.7% efficient solid-state dye-sensitized solar cells

Heewon Bahng, Yiming Cao, Ulf Anders Hagfeldt, Ladislav Kavan, Yuhang Liu, Jingshan Luo, Jacques-Edouard Moser, Yasemin Saygili, Yinghui Wu, Chenyi Yi, Shaik Mohammed Zakeeruddin, Weiwei Zhang

The relatively large voltage loss (Vloss) in excitonic type solar cells severely limits their power conversion efficiencies (PCEs). Here, we report a comprehensive control of Vloss through efficacious engineering of the sensitizer and redox mediator, making a breakthrough in the PCE of dye-sensitized solar cells (DSSCs). The targeted down-regulation of Vloss is successfully realized by three valid channels: (i) reducing the driving force of electron injection through dye molecular engineering, (ii) decreasing the dye regeneration overpotential through redox mediator engineering, and (iii) suppressing interfacial electron recombination. Significantly, the ‘‘trade-off’’ effect between the dye optical band gap and the open-circuit voltage (VOC) is minimized to a great extent, achieving a distinct enhancement in photovoltaic performance (PCE 4 11.5% with VOC up to 1.1 V) for liquid junction cells. The solidification of the best-performing device leads to a PCE of 11.7%, which is so far the highest efficiency obtained for solid-state DSSCs. Our work inspires further development in highly efficient excitonic solar cells by comprehensive control of Vloss.