Nature | Letter

# Efficient planar heterojunction perovskite solar cells by vapour deposition

- Journal name:
- Nature
- Volume:
- 501,
- Pages:
- 395–398
- Date published:
- DOI:
- doi:10.1038/nature12509

- Received
- Accepted
- Published online

Many different photovoltaic technologies are being developed for large-scale solar energy conversion^{1, 2, 3, 4}. The wafer-based first-generation photovoltaic devices^{1} have been followed by thin-film solid semiconductor absorber layers sandwiched between two charge-selective contacts^{3} and nanostructured (or mesostructured) solar cells that rely on a distributed heterojunction to generate charge and to transport positive and negative charges in spatially separated phases^{4, 5, 6}. Although many materials have been used in nanostructured devices, the goal of attaining high-efficiency thin-film solar cells in such a way has yet to be achieved^{7}. Organometal halide perovskites have recently emerged as a promising material for high-efficiency nanostructured devices^{8, 9, 10, 11}. Here we show that nanostructuring is not necessary to achieve high efficiencies with this material: a simple planar heterojunction solar cell incorporating vapour-deposited perovskite as the absorbing layer can have solar-to-electrical power conversion efficiencies of over 15 per cent (as measured under simulated full sunlight). This demonstrates that perovskite absorbers can function at the highest efficiencies in simplified device architectures, without the need for complex nanostructures.

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The achievement of obtaining 15% efficiency within just four years of initial discovery of planar-heterojunction perovskite solar cells by Liu, et al^1^ is laudable and receiving well deserved praise^2^. However, their claims that ?nanostructuring is not necessary to achieve higher efficiencies for this material? and that ?their results demonstrate that perovskite absorbers can function at the highest efficiencies in simplified device architectures, without the need for complex nanostructures.? cannot be supported by their results alone. In fact we believe that quite the opposite is true. With the exception of one, a nanostructured configuration will always have a higher efficiency than the planer cell of identical materials.

In general the efficiency ? is defined as the N~e~ number of electrons produced as a result of N~ph~ number of photons illuminating the surface per unit area per second (photon flux at the surface). Mathematically, ?= N~e~/N~ph~ (1)

Assuming that the loss in light intensity (i.e., the absorption of photons) directly causes the generation of an electron-hole pair, the generation function G(x) at a distance x is given as:

G(x)=?(N~ph~)e^(-?x)^ (2)

Where the absorption coefficient ?=0 if E<E~g~ and ?= A?((E- E~g~)/E~g~ ) if E?E~g~ (3)

Where A is the absorption of light, E is the energy of incident light and E~g~ is the bandgap energy. Since photogeneration will only be caused by photons with E ? E~g~ also N~e~ is equivalent to G(x) in the absence of electron scattering and trapping mechanisms, we can combine equations (1, 2 & 3) as:

??A?((E- E~g~)/E~g~) e^(- A?((E- E~g~)/E~g~) x)^ (4)

Assuming transmittance of light to be infinitesimal and negligible, absorption (A) + Reflection (R) = 1 or A = 1 ? R where R is given in reference^3^:

In that equation in reference

^{3}n~0~, n~1~ and n~2~ are refractive indices of the air, the absorber material and the wide bandgap material respectively, d is the thickness of the absorbing film and ? is the wavelength of incident light.So equation (4) can be rewritten as: ??(1-R)?((E- E~g~)/E~g~) e^(- (1-R)?((E- E~g~)/E~g~) x)^ (6)

Equation (6) demonstrates that the efficiency will decrease as the reflection from the surface increases. Reflection from planer thin films can be minimized to improve the electron-hole generation by adopting a film thickness such that n~1~d=?/4 and thus the minimum reflection R~min~ by resolving expressions in reference

^{3}is given by:R~min~= ((n~1~^2^- n~0~n~2~)/(n~1~^2^+n~0~n~2~))

^{2}(7)The minimum reflection from planar surfaces will always be non-zero in the visible spectrum as given by equation (7) for the combination of materials used by Liu et al^1^ while the reflection from mesoporous, nanostructured materials could be reduced to zero, improving the efficiency.

Strictly speaking, planer thin films are 2 dimensional nanostructures in themselves. However, if the authors were referring to spherical quantum dot (QD) absorbing perovskite nanostructures then we do agree with the authors to the extent that a planer continuous thin film may have a higher efficiency compared to a single particle thick continuous layer of spherical nanoparticles as the volume of material available to absorb light and convert it into electrons would be higher in planer continuous thin film. However, even this may not be true in case of the faceted QDs.

Hence we believe that solar cell junctions of mesoporous wide bandgap semiconductors, cost effectively produced from nanoparticles, covered with a uniform film of perovskite absorber material may always have a higher efficiency compared to that of the same combination produced on planer geometry. Experimental evidence contrary to this will have to be collected through studies performed in the similar fashion as recently reported by Gonzalez-Pedro et al^4^. However, these will have to be conducted with everything else being identical and nanostructuring being the only difference between the devices with enough data collected to prove that the differences are real and do not fall within statistical variation expected.

References:

^{1}Liu, M., Johnston, M.B. & Snaith, H.J., Efficient planar heterojunction perovskite solar cells by vapour deposition. Nature 501, 395?398 (2013).^{2}McGehee, M.D., Fast-track solar cells. Nature 501, 323-325 (2013).^{3}Bauhuis, G.J., Schermer, J.J., Mulder, P., Voncken, M.M.A.J. & Larsen P.K., Thin film GaAs solar cells with increased quantum efficiency due to light reflection. Solar Energy Materials & Solar Cells 83, 81?90 (2004).^{4}Gonzalez-Pedro, V., Juarez-Perez, E.J., Arsyad, W-S., Barea, E.M., Fabregat-Santiago, F., Mora-Sero, I., and Bisquert, J., General working principles of CH3NH3PbX3 perovskite solar cells. Nano Letter, DOI: 10.1021/nl404252e (2014)