To improve the performance of existing perovskite solar cells (PSCs), a detailed understanding of the underlying device physics during their operation is essential.

As a first step, we have developed and validated a device model^[1]  that describes the operation of PSCs and quantitatively explains the role of contacts and of (doped) transport layers, carrier generation, drift and diffusion of carriers and recombination. We fit the simulation to experimental data of vacuum deposited CH₃NH₃PbI₃ solar cells over multiple thicknesses. By doing so, we identify a unique set of parameters and physical processes that describe these solar cells. Trap-assisted recombination at material interfaces (HTL/perovskite and perovskite/ETL) is the dominant recombination channel limiting the device performance and passivation of these traps increases the power conversion efficiency (PCE) of these devices by 40%. Finally, we issue guidelines to increase performance and show that a PCE beyond 25% is within reach.

Grain boundaries (GBs) are ubiquitous in polycrystalline films and are studied extensively in CIGS, poly-Si and CdTe solar cells. The Seto model^[2] is able to successfully describe the GB physics in these solar cells. However, PSCs are different. Perovskites are lightly doped materials and due to the presence of ionic defects it is likely that the traps at GBs are charged when empty and neutral when filled, in contrast to the basis of the classic Seto model. Therefore, a different perspective on GB physics is essential for PSCs.

We include grain boundaries in our model and fit the simulation to the experimental data of vacuum deposited CH₃NH₃PbI₃ solar cells in p-i-n and n-i-p configuration.^[3] Our model quantitatively explains (for both p-i-n & n-i-p cells) the light intensity dependence of the V_OC and FF, delineating the recombination dynamics at GBs and interfaces under different operating conditions. We find that despite the presence of traps at GBs, their neutral (when filled) disposition along with the  long-lived nature of holes leads to the high-performance of PSCs. We also give an estimate of the defect ion density in these solar cells.  

Furthermore, we shed light on the role of charged grain boundaries which may exist under some conditions (under/over stoichiometric preparation). 

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[1] T. S. Sherkar, C. Momblona, L. Gil-Escrig, H. J. Bolink, L. J. A. Koster (submitted)

[2] J. Y. Seto, J. Appl. Phys. 1975, 46, 5247-5254.

[3] T. S. Sherkar, C. Momblona, L. Gil-Escrig, M. Sessolo, H. J. Bolink, L. J. A. Koster (in preparation)