# -*- coding: utf-8 -*-

import numpy as np
from astropy import constants
from astropy import units
from sympy.mpmath import polylog


def uibei(order, energy_lo, temp, chem_potential):
    """
    Upper incomplete Bose-Einstein integral.
    """
    kT = temp * constants.k_B

    reduced_energy_lo = energy_lo / kT
    reduced_chem_potential = chem_potential / kT

    prefactor = (2 * np.pi * np.math.factorial(order) * kT**(order + 1)) / \
        (constants.h**3 * constants.c**2)

    summand = 0

    for indx in range(1, order + 2):
        index = order - indx + 1
        expt = (reduced_chem_potential - reduced_energy_lo).decompose()
        real_arg = np.exp(expt)

        term = reduced_energy_lo**index * polylog(indx, real_arg) / np.math.factorial(index)

        summand += term

    return prefactor * summand


def bb_rad_power(temp):
    """
    Blackbody radiant power (Stefan-Boltzmann).
    """
    return constants.sigma_sb * temp**4


def devos_power(bandgap, temp_sun, temp_planet, voltage):
    """
    Power calculated according to DeVos Eq. 6.4.
    """
    sun = uibei(2, bandgap, temp_sun, 0)
    solar_cell = uibei(2, bandgap, temp_sun, constants.q * voltage)
    return voltage * constants.e * (sun - solar_cell)


# -*- coding: utf-8 -*-

import numpy as np
from astropy import constants
from astropy import units
from sympy.mpmath import polylog


def uibei(order, energy_lo, temp, chem_potential):
    """
    Upper incomplete Bose-Einstein integral.
    """
    kT = temp * constants.k_B

    reduced_energy_lo = energy_lo / kT
    reduced_chem_potential = chem_potential / kT

    prefactor = (2 * np.pi * np.math.factorial(order) * kT**(order + 1)) / \
        (constants.h**3 * constants.c**2)

    summand = 0

    for indx in range(1, order + 2):
        index = order - indx + 1
        expt = (reduced_chem_potential - reduced_energy_lo).decompose()
        real_arg = np.exp(expt)

        term = reduced_energy_lo**index * polylog(indx, real_arg) / np.math.factorial(index)

        summand += term

    return prefactor * summand


def bb_rad_power(temp):
    """
    Blackbody radiant power (Stefan-Boltzmann).
    """
    return constants.sigma_sb * temp**4


def devos_power(bandgap, temp_sun, temp_planet, voltage):
    """
    Power calculated according to DeVos Eq. 6.4. (9780198513926).

    This method assumes fully concentrated sunlight.
    """
    electron_energy = constants.q * voltage

    solar_flux = uibei(2, bandgap, temp_sun, 0)
    solar_cell_flux = uibei(2, bandgap, temp_sun, electron_energy)

    return electron_energy * (solar_flux - solar_cell_flux)

Rewrite devos_power algorithm for clarity