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Sol-Pro Report Summary

Project ID: 647311
Funded under: H2020-EU.1.1.

Periodic Reporting for period 1 - Sol-Pro (Solution Processed Next Generation Photovoltaics)

Reporting period: 2015-07-01 to 2016-12-31

Summary of the context and overall objectives of the project

Light is one of the most important goods on earth: The sun is the single most clean and sustainable energy source which can solve all the energy needs of our world. The energy in sunlight striking the earth for 40 min is equivalent to global energy consumption for a year. During the last years, increasing concern about global warming has led to an intense search for cost-effective alternative energy sources such as photovoltaics. Soluble semiconductors are of increasing interest as new materials for electronic and photonic applications, due to their easy processing, offering the potential for low fabrication cost by printing methods. Solution processed photovoltaic (PV), are lightweight and flexible and offer many new applications, for solar cells ranging from self-powered electronic newspapers to self-sufficient buildings. The ERC project aims to re-design solution processed PV components relevant to printed PVs product development targets. Based on this, processing specifications as a function of the electronic material properties will be established and provide valuable input for printed PV applications. Adjusting the material characteristics and device design is crucial to achieve the proposed high performance PV targets. As a consequence, a number of high-level objectives concerning processing/materials/electrodes/interfaces, relevant to product development targets of next generation solution processed PVs, are aimed for within the proposed ERC programme.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Perovskite PVs have shown impressive progress over the last few years in terms of power conversion efficiency (PCE) but critical issues related to lifetime, large area PV performance and reliability must be addressed. During the first period of the Sol-Pro project a strategy to obtain stable and commercially viable perovskite solar cells is developed. It was shown that low-temperature solution-processed n-type aluminum doped zinc oxide (AZO) carrier selective contact can provide ideal electron selectivity improved lifetime and reliability for solution processed p-i-n methylammonium lead mixed iodide-chloride perovskites (CH3NH3PbI3-xClx) photovoltaics (PVs). Figure 1(a) schematically represents the hole and electron collection process within the p-i-n perovskite solar cell device incorporating AZO. The AZO layer has well matched energy levels for efficient electron carrier selectivity (electron transporting/hole blocking capabilities). Figure 1(b) shows the transmittance measurements performed on AZO layers using the same fabrication conditions as in perovskite PV devices. As it is observed, the transmittance of AZO in the visible light spectrum (~400-800nm) is maintained over 90% even when the thickness of the layer is as high as 390nm. Furthermore, all the three-different thickness AZO functional layers exhibit high electrical conductivity values at very low temperatures. Based on the above the proposed AZO electron selective layer exhibits exceptional transmittance and electrical conductivity values and this provides the flexibility to further increase the thickness of the electron selective layer without losses in device functionality. Thus, AZO can allow the fabrication of relatively thick conductive layers without significant increases in the device internal resistances. The studies performed within the ERC program reveal that incorporation of AZO within the p-i-n perovskite PV structure assists on the electron collection properties via important mechanisms. The thick AZO layer provides an extra energetic barrier for holes and thus improves the electron selectivity of the top electrode. In addition, the smoother interface of the AZO with the top Aluminum (Al) contact significantly improves the reproducibility of the devices by reducing the interface recombination. Finally, the thick AZO layer avoids the direct contact between Al and perovskite spikes and pinholes, thus preventing shunting of the device. Those mechanisms, lead to over 20% increase in PCE for the perovskite PVs incorporating AZO electron selective contact compared to reference (CH3NH3PbI3-xClx) perovskite PVs without AZO. Importantly the proposed top electrode also applied in large area laboratory based perovskite PV devices. Figure 2 (a, b) shows the current/voltage (J/V) characteristics and the photocurrent maps of small and large area perovskite PVs with and without AZO electron selective contact. The effect of the thick AZO electron selective contact it was shown to be very critical for high performance larger area devices. The use of a thick AZO layer within the top electrode ensures a homogeneous photocurrent generation and neutralizes the negative presence of pinholes. Thus, by using AZO on the top electrode larger area p-i-n perovskite PVs can be developed without major losses [Figure 2 (a, b)]. The latter is verified by the photocurrent mapping studies shown in figure 2 (c,d) where the formation of pinholes within the device area is clearly visible for PVs without AZO (Figure 2c). On the other hand, the large area devices incorporating AZO electron selective contact (Figure 2d) reveals a homogeneous distribution of photocurrent and absence of pinholes. These results confirm the beneficial role of using a thick AZO layer also in large area PV devices. Finally, the stability of devices with and without AZO were tested under the ISOS-D-1 lifetime protocol. The perovskite PVs were measured periodically for over 1000 hours of exposure. The lifetime results are presented in Figure 3. Importantly, AZO electron selective contact allows the use of more stable metals than Al, such us gold (Au) to be used as top electrodes (figure 1) and thus highly stable p-i-n based perovskite devices can be fabricated. During the first period of the project we have reported p-i-n perovskite solar cells with AZO/Au top electrode with ultra-stable performance after 1000 hours of lifetime testing under ISOS-D-1 protocol (figure 3). Based on the above experimentally verified observations of the Sol-Pro project, we can conclude that AZO electron selective contacts can resolve major top electrode related barriers for p-i-n perovskite based solar cells. AZO is found to lead in ideal electron selectivity, providing reliable and large area stable perovskite-based solar cells (A. Savva; I. Burgués-Ceballos, S. A. Choulis, Improved Performance and Reliability of p-i-nPerovskite Solar Cells via Doped Metal Oxides, Advanced Energy Materials 2016, 6, 1600285)

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

High performance commercial based PVs require expensive and complicated manufacturing processes involving clean rooms and vacuum chambers. Hybrid solar cells built from thin films of perovskite semiconductors are potentially versatile sources of cheap renewable energy, as they may ultimately make it possible to print large-area solar cells on lightweight flexible surfaces at room temperature. Hybrid perovskite PVs now showing PCE in the range of 20%. However, these cells are facing product development limitations towards commercialization, such as the need to achieve long-lifetime performance and the development of a reliable printed method for the reproducible processing of high-performance perovskite PV modules. By using doped metal oxides, the ERC project has resolved major limitations of printed perovskite PV technology. We have shown that optimizing the design of p-i-n perovskite PV device structure by incorporation of doped oxides electron selective contacts is important not only for achieving higher PCE but also for long lived and reliable large area perovskite PV. This indicates the potential of our proposed PV device structure for mass-producing printed perovskite PVs.

Related information

Record Number: 198119 / Last updated on: 2017-05-17
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