Interfacial Engineering for Efficient Stable and Safe-to-Use Inverted Perovskite Solar Cells


Student thesis: Doctoral Thesis

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Award date2 Nov 2022


Hybrid organic-inorganic metal halides perovskite solar cells (PSCs) are regarded as an ideal candidate for renewable energy applications and have developed rapidly in the past decade. Current interest in PSCs is driven by their remarkable optoelectronic properties, such as tunable bandgap and high absorption coefficients, which propel them to the forefront of cutting-edge optoelectronic research. Recently, the power conversion efficiency (PCE) of PSCs has reached close to 26%, which is competitive with photovoltaics based on traditional inorganic semiconductors such as cadmium telluride (CdTe), polycrystalline silicon (p-Si), and copper indium gallium selenide (CIGS). Despite their attractive performance traits, perovskites have specific inherent material-level challenges.

According to theoretical calculation (Shockley-Queisser model), PSCs still have ample space to improve their performance. One of the main reasons impeding PSCs to reach higher PCE is defects (i.e., point defects, extended defects located at grain boundaries and surface) that form in the perovskite crystallization process (nucleation and growth). Interfacial modulation plays a significant role on perovskite film formation and subsequent morphology, which is crucial in healing defects at perovskite bulk and surface. In PSCs, the functional layers, such as hole transport layer (HTL), perovskite light-absorbing layer, electron transport layer (ETL), and metal electrode, are stacked sequentially to form an entire photovoltaic device. The charge carriers are extracted at the interfaces between perovskite and charge transporting layers (CTLs), which may be trapped by possible interfacial defects.

Apart from developing higher efficiency and ensuring the long-term stability of PSCs, concerns also arises from the presence of lead (Pb), a toxic element that composes about a third of the active layer in the most effective PSCs. The Pb leaking may be caused by the damages of irresistible natural factors, such as flying stones, hail, wind, snow, fire, which may become an obstacle for PSCs in entering the market. Lead is carcinogenic, toxic in small dose, and has a half-life of about 25 years in skeleton. Therefore, it is urgent to address the toxicity issue and develop PSCs that are friendly to the environment for promoting their commercialization.

In this thesis, two major topics are included for improving the performance of PSCs with safe-to-use: 1) Interfacial engineering to mitigate the interface recombination in the interfaces between perovskite layer and CTLs, which is originated from the aspects of energy level, charge dynamics, and defects; 2) Developing barriers by applying lead-absorbing materials integrated in encapsulation architecture to avoid the lead leakage in efficient Pb-based PSCs.

Specifically, a strategy of suppressing non-radiative energy loss at top interface in the wide-bandgap (WB) PSCs was firstly introduced in the thesis. WB-PSCs possess tremendous potential for supplying stable power sources for Internet of Things (IoT) indoor application, since their absorption range are well-matched to the indoor light spectra. However, the development of WB-PSCs is hindered by non-radiative energy loss at perovskite interfaces and photoinduced phase segregation. To address this issue, a simple and effective passivation strategy was proposed by applying phenethylammonium halide treatments (PHTs) on WB perovskite interface. After being incorporated with PHTs, the interfacial recombination caused by high density of trap states were significantly inhibited, supported by the prolonged lifetime (time- resolved photoluminance, TRPL) and enhanced steady-state photoluminance (PL) intensity. Besides, the phase segregation on the perovskite surface was also suppressed by PHTs, enabling a more uniform interface supported by the result of PL mapping and continuous PL measurements. Combining these synergistic effects, the resultant devices could reach a PCE over 18.3% (1.75 eV), with a record open-circuit voltage (VOC) of 1.26 V. Meanwhile, the device's photovoltaic performance under indoor light has been greatly improved, leading to an indoor PCE of 35.6% when illuminated with a white light-emitting diode (LED) at 1000 lux.

The successful interfacial modulation on single-junction PSCs has also fueled research on devices with tandem structures, which couple perovskite photovoltaic with different photovoltaic materials. Due to their potential as highly efficient and flexible photovoltaic devices, perovskite-organic tandem solar cells are gaining more attention. In the second work, the WB perovskite was integrated with low bandgap organic photovoltaic (PBDB-T:SN6IC-4F) to form an efficient perovskite-organic monolithic tandem solar cells. A high VOC of 1.22 V was achieved on single-junction WB-PSCs by employing phenmethylammonium bromide (PMABr) to passivate the perovskite surface. Taking advantage of the efficient WB-PSCs, a perovskite-organic tandem device was assembled, leading to a satisfactory PCE of 15.13%, with a high VOC of 1.85 V.

To further understand the impacts of the interface on perovskite, interfacial engineering with respect to material choice was systematically studied. In my third work, a ferrocene-based organometallic interface material (FcTc2) was developed, which combines the advantages of organic interface materials (OIMs) and inorganic interface materials (IIMs) for high-efficient PSCs. The carboxylate and thiophene side arm in FcTc2 provided robust chemical Pb-O binding to reduce surface trap states caused by the uncoordinated Pb2+. Meanwhile, accelerated interfacial electron transfer can also be achieved by the electron-rich and electron-delocalized ferrocene framework. The synergistic effects of enhanced interface binding and accelerated carrier transport by FcTc2 enables superior device performance and stability. A PCE of 25.0% (with certified 24.3%) in inverted PSC was firstly achieved, along with superior long-term operational stability (maintained > 98% of its initial efficiency) under continuous 1-sun illumination for > 1,500 hours. Furthermore, the FcTc2- treated devices demonstrated excellent endurance under damp heat environment (85 ℃/85% RH), which have passed the international standards for silicon solar cells.

The last part of my work focused on minimizing the risk of environmental lead contamination for lead-based perovskite photovoltaics. Sulfonated graphene aerogels (S-GA) were first assembled in flexible perovskite solar modules (PSMs) encapsulation structures. The large specific area and high binding energy with Pb2+ of sulfonated graphene aerogels enable them superior lead adsorption capacity in an aqueous solution, which is a promising candidate to integrate in poly(methylhydrosiloxane) (PDMS) as encapsulant for perovskite photovoltaic. To examine the lead-absorbing capability of the encapsulant, different conditions such as scratching, bending, and thermal circling were conducted to simulate the daily PSMs application scenarios. The encapsulant can capture over 99% of Pb2+ from the degraded flexible PSMs, the lead leakage was significantly reduced to ≈10 ppb, which is far below the hazardous waste limit according to the resource conservation and recovery act regulation (RCRA).