Although the power conversion efficiency (PCE) of Perovskite solar cells (PSCs) has reached record high 23.7 % [1], poor stability of PSCs in natural ambient conditions [2-4] obstacles their implementation in real functional devices. To date, huge efforts have been made to develop PSC devices with superior stability. The aim of this presentation is to present a new technique for robust encapsulation of PSCs, which render the devices with superior stability ever reported.

Apart from stable materials for PSCs, robust hermetic encapsulation of the cells is a promising pathway for overcoming stability issues related to the interaction with the environment. Sealing with glass frit is a well-known industrial technique allowing the best hermeticity of the sealed devices; requires, however, high process temperature (> 380 °C) [5], not compatible with PSC. Localized laser-assisted sealing can be used to encapsulate PSC devices with glass frit at much lower temperatures (< 130 °C) [6].

Commonly, PSCs include an electron transport layer (ETL), perovskite light absorber and a hole transport material (HTM). Titania as ETL, HTMs such as spiro-OMeTAD and PTAA are the most commonly used materials in PSCs with high PCEs. An industrial PSC device must pass a series of standard tests, including withstand temperatures up to 85 °C [6]. Perovskite crystals are thermally stable up to 130 °C, however, organic HTMs degrade at 85 °C [7]. Since, perovskite possesses an ambipolar charge transport, HTM-free PSCs can be produced [8]. HTM-free PSCs have the advantage of high thermal stability and scalability (i.e. screen printing deposition). Here, we present a cutting edge laser-assisted glass frit sealing technology for encapsulation of HTM-free PSCs.

HTM-free PSC devices (FTO/bl-TiO₂/meso-TiO₂/meso-ZrO₂/C, (5-AVA)_x(MA)_1-xPbI₃) were encapsulated at ca. 100 °C. The encapsulated PSCs displayed a He leak rate (2.4 × 10^-9 atm cm³ s^-1 He) lower than the reject limit of hermeticity test method of MIL-STD-883 standard. The encapsulated PSCs were further subjected to 70 thermal cycles (-40 °C to 85 °C) according to IEC61646 standard and 500 h at ca. 70 % RH stability tests. To compare the effect of hermetic sealing, cells with no-encapsulation and not-hermetically sealed were also subjected to the stability test. The stability to humidity studies showed that the PSCs with no-encapsulation and not-hermetically encapsulated lost their performance after 50 h and 150 h, respectively. In contrast, hermetically sealed PSCs displayed negligible losses in the PCE (ca. 5 % of the initial value) after 500 h. Moreover, the hermetically encapsulated devices retained ca. 93 % of their initial performance after 70 thermal cycles.

To date, most of the sealants (e.g. epoxy and thermoplastic) used for PSC encapsulation are neither hermetic nor long-term stable. To the best of our knowledge, this is the first report on hermetic encapsulation of PSCs. The present study suggests that the prospective commercialization of PSC technology would use low process temperature laser assisted glass encapsulation.

The PCE of HTM-free devices are relatively lower comparing to HTM-based PSCs. The authors are now improving the glass encapsulation process for sealing at ca. 85 °C; this will allow to encapsulate inorganic HTM-based PSCs (e.g. CuSCN), which display substantially higher PCEs.